my 2 cts after experimenting both RS and GS (not 1st row) 2 nights in a row :
RS was cool because you can see the whole stage and set, but the production is so amazing than if you're not prepared, you can miss a few things being in RS (can't say more without spoiling, but let's say she MIGHT be popping from place to place without you noticing...), but i was with people who'd seen the show so they were telling me where to look or where to go... I must say i had a feeling of running a marathon, but you don't HAVE to go EVERYWHERE she stands if you don't want to run around RS... You could just stand in the middle and turn around and enjoy everything. And dance, cause really, PLENTY of space.
Now GS : i was not front row but arriving at opening doors without queueing all day, i was maybe between row 5 and 6 at the tip of the prism. Something like that, still close to the front row... Maybe there was still room closer to the front row but on the side of the catwalk, but i was told that center (top of prism) was the place to be (i get where that comes from since it's the 2nd stage of the show, as catwalk is MAINLY - but not only - used to go from stage at the back to stage at the front). BUT i'm not tall (not small, but not tall) and couldn't see much of what happened at the tip of the prism (or at the back of the stage, actually, and not at all on the catwalks) cause the tip-of-the-prism stage is lower than a usual stage (it's about chest high) and the back of the stage is so far back... But during the acoustic set i could really see her and enjoy it, between people's arms and cameras :-) i ended up looking at the screens quite a lot other than during the acoustic session.
Both zones have inconvenients and advantages, and seats are really great if you're not tall and not seeking eye contact during the 2 hours of the show (or if you can't access floor standing because of age).
I think if you can't be front row GS, you're not tall and you don't particularly seek eye contact, opt for the best seats you can afford (but all would have perfect view, it's more a question of "what size do i want to see Katy ? Human size or ant size ?").
If you can queue all day (be 1st or 2nd row), or have early entry GS, GS is really great.
If you want eye contact + stage view + not queue all day, choose RS. But you'll have to see her (nice) back sometimes.
I don't know if it changed since the 1st shows, but she does look at RS a lot, during all the show except A PART of the acoustic session (with the guitar and the chat to the audience)... But she doesn't spend the last 45 minutes with her back to RS, really didn't experience that (i remember reading it in the beginning). Maybe she adjusted that from shows to shows...
My 2cts ! but US will be different as there's no standing outside the RS so everyone should see quite well...