So, I'm building up some training wheels, grabbed some Askiums from the Performance Bike going out of bidness sale. They were still selling cassettes for much higher than what you can get a Ultegra R8000 on Amazon.
The cassette that came with my Argon 18 E119 is an R8000 11/28 and there are a lot of other options I've noticed and it seems clear as mud. In the back of my mind I'd think an 11/32 gives more range but from what I can read that may not be the case.
There's nothing complicated or mysterious about this. 32 teeth is more than 28 teeth and they both have 11 at the other end. An 11-32 does give more range than 11-28. Pretty simple.
The only reason you may not get more range via a simple cassette swap is if something else in your drive train was preventing you from using those extra teeth. For example if your rear derailleur doesn't have sufficient capacity or your chain is too short.
However, more range is not necessarily better. It depends on such factors as where you ride, how you ride, your ability, your weight, whether you're willingness to change setup between rides, whether you favour nominal setup for most common routes over flexibility of setup for all potential routes, etc.
I occasionally ride my Tri bike in the mountains but usually bring my road bike. I don't mind switching cassettes for special occasions but don't bother doing it every week. I'm fairly heavy for a triathlete (usually over 80kg, except at my leanest mid season on good years!). I don't mind a low cadence but don't like being forced to high intensity on long hills late in a long ride just because I have insufficient gears to do otherwise.
I used to use 50/34 and 11-28 on my road bike but put on a 11-32 cassette for climbing Gallibier, Glandon and Alpe D'Huez in the Alps last year. It turned out I didn't need that gear but it was nice to have it just in case my knees started suffering. I planned to switch back when i got home but actually never bothered. With 11 sprockets, I don't find myself missing the smaller gear ratio spacing and that small gear, though not essential, does come in handy now and again on steep climbs.
On the Tri bike I've got 52/36 chainrings and generally use 11-25 as it spends most of it's time on the trainer or riding flat or rolling routes. However I've used 11-28 in the past for hillier courses and it is only a 5 minute job to switch.
Unless you're new to cycling, or plan to ride a route that's dramatically different to what you're used to, surely you already know what gears you want/need?
Bear in mind that so long as you have the ability to switch cassettes, and if you do a lot of riding, there may not be much additional cost to having more than one cassette. You'll be spreading the wear and tear around and they'll obviously last longer if you're using them less because duty is shared.