The servers for Tri will be going down at the end of April, which will cut off 60% of the content. If you have the money to spare you should probably get 3U, but if you've never played a MH title before then you should first drop the 10$ on this and figure out if 3U will be worth 350.
If no, you have to consider if it's worth the money paying for a console/handheld and the game. If there are a few others games on the system that interest you, and you are positive that you may get one down the line, then maybe wait a while
However, if you are new to monster hunter, and want a quick experience, I suggest getting Tri.
Though you'll only have access to 40% of the game by the end of April, there are still a lot of hours that can go into it.
Don't play to win, play for fun. Love playing games with people, that's why I love Wi-Fi 3DS: 1203-9417-6170, PSN: Trixster196
It's not worth it to get Tri with the servers closing soon, honestly. Online is a huge portion of the game, and the game was already anemic in content compared to its siblings.
If you have a 3DS or Wii U, MH3U has triple the content of Tri, straight up. It's also the one most packed with people to play online with if you're into grouping with randoms (or people on GameFAQs, which is pretty much the same thing.)
If you have a PSP, you have access to MHP3 (with translation patches) and MHFU.
If don't have them and you aren't ready to commit all of your Bison dollars to a new console that isn't proven yet, the PSP is still fairly cheap, and has a well-proven stable of pretty decent to downright good games.
If all you have is 10 bucks, then sure, get Tri as a.. Tri-al? (Ha ha ha kill me now.) You'll probably get 10 bux worth of fun out of it.. but it's a better idea to dig in to one of the other games.
Contrary to what the others are saying, if you're just looking for a single-player experience, then Tri can deliver on that.
From start to end of the offline story can be done in a few days if you're new to the game and have time to devote to it, but once you get into looking into the details of armour skills and weapons, you could easily devote weeks to grinding out quests in order to get a particular setup.
If you go in thinking that there's no point to it thanks to online servers being shut down in a few weeks, then you'll not get much out of it. If you don't give two hoots about online and just want a decent enough game to play, then Tri can certainly deliver on that.
Where logic won't work - I will PAL Gamers unite - http://www.gamefaqs.com/boards/277-pal-gaming
If you have a 3DS or WiiU, get MH3U. Theres just so much more content, and Tri pales in comparison. Even with the 3DS version that can't go online unless you have a wiiU, you can still do all of the online quests by yourself or with a friend via local multiplayer.
I loved Tri, but Tri ultimate is just the better game. Already put 125 hours into it, and its only been a couple of weeks, and I haven't even reached G rank yet.