Cassette question 11/25 vs 12/30

11-28 is my go to hilly route cassette.  Higher than that you might need a different rear derailleur (not 100% sure on that) and/or a longer chain.

Most any rear derailleur will handle the 11-28 and a 50/34 compact. I've done it many times with short-cage Ultegra and 105. And with some fancy fanagling (like reversing the B-tension screw), even an 11-32, but that's dicey (and better served by going to a long-cage off-road derailleur).

i run a compact as well and regularly switch between 12-25 and  12-30.  I might even have a 12-23....  next cassette I buy will be an 11-28 though

I would check the specs on your rear derailleur.  You can force it to work by adjusting the B screw, but you lose the ability to cross chain.  I don't recommend cross chaining for extended amounts of time, but it's nice to have the options in those situations where it's only needed for a few seconds.  Because those are the situations where you would have to otherwise shift the front...then shift it right back.  Kind of irritating.

That said, I use a 52/36 up front and a 12/32 in the rear for training on my road bike.  52/12 is plenty of top end for training as I don't generally bomb down hills.  36/32 is a nice bail out gear on really steep climbs, or really long days in the saddle where 7% climbs start to feel like 12% at the end of the day.  The "jumps" in the cassette is the same as 11-28.  You swap the 11 for the 32.