I doubt either is any faster than the other. The Felt comes with the calpac storage on top tube, as well as the new draft box storage behind the seat tube. Plus the felt looks nicer IMHO. So, I'd vote felt for sure.

If you get the felt make sure the aero bar clamp is the new one. There are some problems people had with the clamp shearing off and FELT has not addressed the problem with recalls. Make sure the clamp has V3 on it. If it has V2, you can get it replaced for free at your LBS. Just make sure its V3. V2 have caused some crashes (one guy on here was really hurt)

Felt IA 10

Pros:
+ Integrated storage
+ Clean cable routing for rear brake

Cons:
- Slightly Heavier
- Difficult to service rear brake

Cervelo P3

Pros
+ Easy to adjust brakes
+ UCI legal (if you want to do TT races)

Cons:
- Comes standard with external DI2 battery. Your LBS can change that and fit one inside the seatpost though.

I have an IA10 love the bike hate that bottom bracket brake....honestly I would say buy the P3 (after I just said I love my Felt). Really its about that bottom bracket brake.

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"see the world as it is not as you want it to be"

Replace it with a Shimano Ultegra direct mount (very good and economical) or a Tririg Omega X (spendy, but freakin' excellent). Totally changes everything.

P3. Easy to work on, lighter in weight, no slower.

If Cervelo, check to see if a shop will place Di2 on a P2 for a similar price or, more likely, simply swap out the aero bar for something better like the Profile Design bar that comes on the P2. Otherwise, the Felt offers a larger fit range including independent tilt adjustment of the pads and extensions. All their new bikes come with the V3 clamps, so that's not an issue.

Brakes on the Felt are better, and the front Vision aero brake is what would make it a "faster" bike out of the box. I don't understand the issue with the rear brake. For 2018 it's the 105 direct mount and easy to set up. Works great. Last year, some of the IA's had Shimano direct mount, some had the FSA. I didn't like the FSA as much. I'm still not a fan of the head tube compartment for the Di2 junction box, but it's a little better this year with the 3-port junction box instead of the 5-port from last year. Though, now that I think about it, the 48 doesn't have that compartment, so better to put the junction box in, or under, the Calpac anyway.

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Jim Manton / ERO Sports

I put an eecycleworks brake on my IA10 the day I got it from LBS when they first came out and once it was set I haven’t had to fiddle with it more than a couple of times switching from 808 to disc. It’s been almost 2 years. Eecycleworks is expensive but it is great hardware with superior engineering.

I have the Vision aero front brake which I really like the bottom bracket brake is FSA...and a pain in the you know where....just sayin' by whats the real difference between Rotor Q-rings vs. QXL rings? :)

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"see the world as it is not as you want it to be"

The two bikes have totally different geometries. I have a hard time believing that if she fits well on one, she will fit well on the other. I say this as someone who is your wife's size (more or less; I am size 48 on the P3 and XS on the Felt IA10). I test rode both bikes. The P3 was way too stretched out for me. The Felt was perfect. Ended up on the Felt. But the point is, the geometries were WAY different.

Having said that, if she feels great on both, I would say go with the P3; it's a smoother ride than the Felt.

Another option is to buy a P2 with the Ultegra Di2, which is what I just bought for my wife. My wife tried the electronic shifting and she was sold. It isn't a very common model so it may be hard to find though. Pretty good value since it's "almost" a P3 but with Di2 for a lot less money.

Reach is the same on both bikes in size 48cm; stack is different by 2cm with the Felt being lower, but as long as she can achieve her pad stack, these bikes are actually quite similar.

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Jim Manton / ERO Sports

I agree that, on paper, they look very similar (stack 467 Felt vs 485 P3, reach 380 Felt vs 381P3), but what I'm saying is that they feel totally different when a woman for whom they are made sits on them and tries to ride them. I don't know if it's a stack issue, a wheelbase issue, a seatpost angle issue, or something else, but when I test rode both, I felt really stretched out on the P3 and really good on the Felt. n=1, but given that I'm about the same size as the OP's wife, I thought I'd offer my input.

I'm considering an IA myself, could anyone share if the rear brake interferes with a stages powermeter? I had to get a different brake for my AR

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USAT Level II- Ironman U Certified Coach

Thanks to all for the opinions, suggestion and overall help. Lots of great info.

Went with the P3 and relocating the battery inside the seat tube.

Should have it in 2 weeks.

Were they setup the same and the P3 was simply out of your fit range, or?

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JustinDoesTriathlon

Owner, FuelRodz Endurance.

Not that relevant tbh. The P3 weights 8.8 kg, and you can make the IA10 weight 8 kg, if you replace the stock wheels and cockpit!
The IA10 is also compatible with Tri-Rig posts, not sure about the P3

Ps: ah seems like I'm late to this :)