I know you're talking about "lines of phones", but even if you buy the nexus today it will be technically "outdated" very soon, so whether you get an htc or a nexus doesn't matter. If you really care that much about being outdated, you shouldn't get a phone at all.
Considering HTC has been the best about upgrading/supporting their phones, i think getting an HTC phone right now is a safe bet.Sensation is getting old, it's getting ice cream sandwich. Even the thunderbolt, at the very least rooted, will get ice cream sandwich. Hell, i was pretty shocked at the 1st gen incredible getting gingerbread.
Personally? I buy my phone for what it does now and not for what i fear it may not do a few months down the line. Consumers get so upset when they buy a phone and a new one comes out a month later with slightly better specs. So? Slightly better specs doesn't change the fact that your phone does what you bought it for. I don't expect the entire industry to slow down because i want to say "i have the best specd phone" for awhile longer.
Seems like all we ever focus on are numbers on paper and not the fun/experience we actually have with the phone. Nevermind the fact that most people play basic games like angry birds or don't even shoot 1080p cell phone video. We need quad cores soon. Right?
The nexus will only be outdated soon if you completely erase the "experience" of the phone from your brain and only focus on jerking off to numbers on a sheet of paper. Or well, unless you really need that much power in your phone. In which case, you're probably a power user and upgrade every year anyway right? Can we ever just sit back and enjoy our god damn phones without being envious of whatever comes next? . My first phone couldn't even handle mp3 ringtones