I have an S4 mini, as the S4 fathers an iPad in its size!
The phone itself IMO is slower to respond and process inputs, eg opening the camera app, typing quickly in an SMS etc, than my old S2 it replaced, but its not horrifically slow so dont let that put you off. I've had it about 3 months and so far (touches wooden side table) it hasnt done anything wrong or locked up etc
Cant really comment on the HTC but my friend had a full size HTC One for 12 months and he has replaced his with a Blackberry rather than another HTC, he didnt seem keen on sticking with the brand but didn't gave a proper reason why.
It paired up first time no probs at all with my e86 3.0si coupe bluetooth