Different size cassette for the trainer? Like I hoped, after a few months of hunting, I was finally able to find a really good deal on a road bike, in the winter. With the money saved, I was also able to buy a smart trainer to use with Zwift.

I need a spare cassette for the trainer. My current setup that came with the bike is 105 CS-R7000, 11-30T I'm thinking of getting a second cassette with a different range - I can see that they generally all start at 11 teeth and can even go up to 34T, but actually I'm thinking of getting 11-28 instead as where I live is pretty flat and i don't expect to use the 30T at all.

Should I get a different size cassette? Which one should I use on the trainer (given that Zwift has Alpe d'Huez etc) and which one should I keep for my road use? Is there any point in getting Ultegra (my bike is 105) and I clearly don't care for the weight on the trainer, but I've heard reports that Ultegra is more durable?
Should I get a different size cassette?


Probably.  You'll need one eventually, as they do wear out.  


  Which one should I use on the trainer (given that Zwift has Alpe d'Huez etc) and which one should I keep for my road use?


Having a cassette more closely-spaced than 11-30 is probably good for a trainer, as you can more closely target specific power levels and cadences - as long as they're within the narrower range of a tighter cassette.


  Is there any point in getting Ultegra (my bike is 105) 


IMO, no.  I've raced on Tiagra cassettes as well as 105 and Ultegra, and I never noticed any performance difference between even that cassette and any Ultegra or 105 cassette.  And I have quite a few of both - 11-23s, 11-25 (IIRC, that's the Tiagra one), 12-25, 11-28s, and 12-27s.  I'll swap cassettes as needed.  When racing, I'd chose the narrowest cassette that, if possible, allowed me to stay on the large chainring the entire race.  For training, I tend towards wider cassettes such as 11-28 or 11-30 to enable hill climbs at low to moderate levels of effort, should that training ride be a longer, lower-power ride.


  and I clearly don't care for the weight on the trainer, but I've heard reports that Ultegra is more durable?


Meh.  In my opinion and in my experience, the only real difference between 105 and Ultegra cassettes is the color.  Ultegra is darker.  The minimal weight difference comes from 105 cassettes having a steel lock ring, and Ultegra cassettes having an aluminum lock ring that cuts an entire gram or two from the weight.

So no, don't spend more for an Ultegra cassette over the same 105 cassette.  Only spend more if you need a specific gear range only available in Ultegra.