If you're also a fan of D&D3/d20 you should definitely buy the T20 stuff, it combines the two systems better than anyone would've expected.
If you're looking to actually play Traveller with new players then you should probably buy the T20 stuff because it'll be a much easier sell than CT (d20 is BY FAR the most popular/familiar current rule system, and players today are (inexplicably) afraid of learning new rules, especially "old" new rules).
If you no longer have your old Traveller stuff and are looking to get back into the game then you should maybe buy the T20 stuff because it's in print, it's a good starting point, it's mostly true to the feel and spirit of CT, and it's being actively supported (but note also that most of CT is now back in print due to FFE's Reprint Series, so you might consider picking those up instead).
However, if you've got an existing group of CT players, still have your CT stuff and don't necessarily care about actually playing (i.e. are a grognard), and/or don't like (or don't know/care about) D&D3/d20, then there's no real reason to buy T20, depending on how big a hit the $45 pricetag feels like. The rules IMO are still nowhere near as good as "real" Traveller (by which I mean CT and MT) and, until the Gateway Domain Sourcebook is released, there's been no new background material of any substance. I bought it, 'cause I'm a collector (and 'cause as a preorder it was way less than $45!), but it hasn't been off my shelf once since I finished reading it.