Both of them really are great d20 games, and both are worthy of purchase.
SW is, as noted, quite specialised. I wouldn't recommend it for playing anything but SW. OTOH, if that's what you want to do with it, or if you want to get your brother interested in RPGs, I'd highly recommend.
What I would NOT recommend is the Rebellion Era Sourcebook. It's an overpriced waste of cash, IMHO, and cash is probably what you're short of.
You would be much better off getting the Bill Slaviscek softback Star Wars encylopedia (not the big colour job, it's a small B&W thing, least it was when I bought it). It focuses on the Rebellion Era and has a lot of background detail and ideas, and is great for looking up things mentioned only passingly.
Other than that, the SW Chronology, if I'm remembering the name right, was a very useful book to me in designing my campaign. It's part of a series of books covering aspects of the SW universe (isn't an RPG book), and has a detailed chronology (duh) of the SW universe and EU, and contains lots of hooks and ideas, quite accidentally.
The good thing about SW Revised, though, is that the core book really does "stand alone", decently, you don't *need* any add-ons to play a typical SW campaign, especially if you're willing to make up ship statistics, etc.
Of the official, WotC RPG books for SW, I'd recommend only Powers of the Jedi, and only if you'd be likely to have a Jedi-heavy campaign. It contains some nice new powers, and has a great deal of detail on virtually every Jedi from both the movies and the EU. A lot on Jedi from Ep.1 and 2, though, so...
The Dark Side sourcebook is alright, but out-of-date, and unecessary for most campaigns.
The equipment and ship books, for my money, add very little to the game, except detail, and I'd only recommend the equipment book for a Bounty Hunter-heavy game (where all those neat gadgets, wierd weapons, jetpacks, speederbikes, etc. might make a difference), and the ship book for, you guessed it, a piloting-heavy game.
All the era/planet sourcebooks so far seem to be pretty bad, and I'd stay away. In particular, the era books contain very little "hard info" or "crunchy bits", and a great deal of chronology, which you can get all of in the SW Chronology, for quite a bit less.
SW is thus pretty modular. All the sourcebooks are only useful if your campaign focuses on one area or another, thus you should just pick up the core SW Revised book, and think about the others later.
On Traveller T20, it's a wildly underrated game, very good in most respects, but it's a very different game to SW, and unless he's into the Traveller universe, or '70s space SF (Larry Niven, for example), I wouldn't suggest it.
