Since you don't need TV and physical DVD, the PopcornHour is probably well suited for you. I don't think the containers and the codecs it supports will become obsolete that soon that you have to worry about that.
I have a Popcornhour A100 and I'm reasonably happy with it, but it does have it's share of mildly annoying bugs, nothing that's a total deal breaker for me though.
The main advantage is, it's mostly plug'n'play (unlike a HTPC), but if any of it's flaws or limitations annoy you, then you can't do much about it as you don't have much control over it's software.
A HTPC means you have a lot more flexibility with regards to software, but that comes at a the cost of sometimes long frustrating configuration sessions until you get everything working to your satisfaction (and there is no guarantee that you ever reach total satisfaction!).
A year ago when I bought the PopcornHour the choice was a no brainer, it was much cheaper, smaller and more silent than a similar capable HTPC.
These days with Atom/ION systems such as the Asrock ION 330, the choice is not so obvious anymore and in fact I'm seriously considering to replace my PopcornHour with an ION HTPC, but I'm still waiting for more ION systems to be released.
I hope my rant is of some help for you rather than making you even more confused, I'm undecided myself so can give you a clear recommendation!
But I would second what zim2dive suggests, try out any HTPC software you are considering, on a normal PC first to see if you like it, that's what I'm currently doing, I'm testing various HTPC software on my desktop PC (the fact that I have VDPAU capable GPU in it helps getting a good impression on how it would work on a HTPC).