Do you race full Ironman distances? Yes=920XT; No=735XT
Is a barometric altimeter important to you? Yes=920XT; No=735XT
Do you want a quick-release for bike mounting? Yes = 920XT; No=735XT
Do you want WiFi syncing, or is Bluetooth good enough? Yes = 920XT; No=735XT

I do race full Ironman...why do you note the 735 isn't sufficient?

The big limiter for the 735XT in an IM is battery life. Depending on your anticipated finish time you might need to adjust modes (like using a HR strap instead of the optical HR) to cover the length of a full IM.

I raced a 70.3-distance triathlon today and my 735XT ended up just below 50% battery when I finished, but everything was running: GPS + glonass, optical HR, bluetooth was still on. I'm working towards a full IM next year and I'm not worried about it, will just adjust settings on race day to ensure I have some buffer.

What does just having blue tooth syncing limit? (rather than both blue tooth and wifi)
Thanks!

Online it notes 14 hrs.

I'm roughly 11:30 or better - do you think the 735 would do the trick?

Also what is the limitation for just Bluetooth vs Wifi and Bluetooth?

WiFi is just another path for syncing. To me, it is not an important feature, because BT syncing works fine. But to some, WiFi is a big deal.

I forgot another key differentiator. The 735XT has the built-in HR monitor. And in general, the 735XT is designed for 24/7 wear as an activity tracker.

11:30 should be no problem, but I would keep bluetooth off on race day (unless you are pairing to bluetooth sensors on the bike) and probably turn off glonass on the GPS just to be safe.

I think you would be on the bubble with a strong probably. Garmin states "Up to 14 hours," and DCRainmaker tested it pretty close to the rated time. But, that is straight off the charger with probably some features disabled. If you think about real life use, you will probably pull the watch off the charger at least three hours before race start, maybe even earlier. So, it will have a little battery burn before you get to the start gun. Then, you may not want to fully compromise the full features of the watch. With your projected time, you would probably be OK, but the 920XT would give you total confidence with no features disabled (albeit, you would have to use an external HRM with a 920XT).

I've had the 735xt on my wrist almost since it first came out. I love it, but if I had to buy again, I'd probably get the 935xt.

I race Ironman 70.3's exclusively, and I'm ready to pull the trigger on the 735xt. I probably would even if I was to race an Ironman 140.6. I'd just have certain features disabled. The form factor did it for me. My buddy has a 235 and the 735xt is the same size. It's quite large. I've watched videos of the 935xt and it's bigger. When you add the quick release onto the watch it becomes a small tank. I plan to use the Garmin Bike Mount on my Profile Design Aero HC hydration system, so I can double the use as a bike computer as well as for racing.

Swimming and running with a small tank on the wrist is not my idea of a well designed multisport watch. The quick release feature would be a bonus, but the design makes it a non-starter for swimming and running. It's not my style and I'm all about improving efficiency and speed.

735 weaknesses:

battery life
no barometric altimeter
optical HRM sensor sticks out from below and some find that uncomfortable on their wrist
no wifi - that could be a non-issue or a big deal depending on expectations

935 solves all those, and it is only a tiny bit larger than a 735.

735 was available for $300 right around the time 935 came out. It looked as if that was a permanent price drop, but now it is $400. Frankly it makes no sense to buy a 735 at that price point.

As for 920 it is a great watch and that's the one to get if you don't need optical HRM.

Great list. I would add:
Do you want a watch that screams tri-dork? Yes = 920XT; No=735XT

Before considering the 735, I'd check the price difference to the 935. I know it used to be super small before they dropped the 735's price. Now, I believe they are different enough to actually consider the 735.

The 935xt with the quick release is significantly larger than the 735xt which is a pretty good size itself. The 935xt is a tank with the quick release.

"Tri Dork" is a great description.

Did you end up getting the Garmin 920? Is it still worth buying with the 935 out there?

735 battery life will be fine. I had 30% battery after a 9h30 IM. The lady is an 11 hour racer and hers had juice after as well.

Agreed - I got a 735 for $265 right when the 935 came out (thanks eBay coupons) and at that price it was a steal. At $400? The 935 has a better battery life, BT sensor support, a better optical HR monitor, and barometric altitude for $100 more. Plus you'll probably get at least a year more worth of meaningful Garmin updates.

The biggest advantage of either the 735 or the 935 over the 920 is going to be the internal hardware is faster. I had the 920 before and while the 735 isn't smart watch fast, it's generally much better from a navigating screens perspective. Also, if you want it as an activity tracker, the 920 is a tank (size-wise). FWIW - I never got the Wifi syncing to work on the 920, but Bluetooth on the 735 has been pretty smooth.

You should consider the size of your wrist.

Also, no one has mentioned the weight difference.

Personally I'd eat rice for a few weeks and spend the money saved on a Fenix5 or 935, at the very least for the better battery.

I have been using a 735XT for a little over a year. Positives: Wrist HR - some say it's not as accurate as a chest strap, but it's good enough for me and I appreciate the convenience of not wearing a strap. Size, look & feel - It fits under (most) of my shirt sleeves. Because appearance is much like a "regular watch", I can wear it all the time. Watch features - Decent customization features regarding what screens you see and what is on them. Good zone alarms. Has a triathlon mode. Browser & App UI Features - Good enough, at least the app. I like the beta app UI that is currently in final testing. Desktop browser UI sucks. Polar Flow is much better (I previously used a Polar M400). I use Training Peaks on the desktop instead of Garmin. Negatives - Battery tops at about 10 hours with everything running. Doesn't get me through an IM, but YMMV. Next time I'll turn off the GPS but retain the HRM.

The bluetooth vs wifi, for me, is a non-issue. As soon as the watch is within range of my phone, it syncs via BT. Then Garmin Connect on the phone updates Training Peaks, Strava, etc. automatically and immediately.