Well LDS, it just goes to show how subjective phone impressions are. The power button location isn't a problem for me, I can easily operate it one-handed in my left-hand, when using my Lumia 920 I always appreciate the slimness when back on my One, etc. etc. I had a bit of a learning curve on how Sense 5 likes to do things, but after a couple days of use it's already second nature so doesn't bother me at all.
My one question is why your unit has poor performance....mine scrolls up and down flawlessly in Sense. Internally some *apps* may be a bit slower, but the OS has been nothing but silky smooth for me, so I wonder if there's something running amok in the background on your phone to cause that?
However, it sounds like you're not "clicking" with the phone, so whether or not it's worth tracking down the cause of the performance issue is up to you, you might just want to go for the S4, especially if you like the S3 lots! And, btw, why didn't you get your wife the S*4*?!? Some nice advantages over the S3 like screen brightness and a better camera. I'm sure she'd prefer it!