I assume we're talking 2011 models?
If Raleigh still offered the same RockShox Tora fork and Deore-level componentry on the Talus 8.0 that it did on the 2008 Mojave 8.0 that I own, I'd urge you to take a hard look at the Talus. But Raleigh has progressively watered down the componentry on its 8.0-level bikes so that the 2011 Talus now has virtually the same components as the Trek 4500 disc.
Both have identical Alivio drivetrains and similar Suntour forks. The Trek has Shimano hydros instead of the Raleigh's Shimano mechanicals, but I have no time with either so I can only assume the hydros are better.
If the bikes are being offered at the same price, and if the fit and feel on both bikes is relatively the same, I'd recommend the Trek. But if you can get a good deal on the Raleigh, then maybe that's the bike for you.
Still, your first and foremost criterion should be fit. Which bike feels best to you in the saddle? If the price difference isn't significant, go with that one.
I purchased a 2008 Raleigh Mojave 8.0 hardtail--the Talus 8.0's predecessor--for $550 in January 2008. I have put literally thousands of miles on it and it has treated me very well. I did have to replace the frame last year but Raleigh handled that quickly and without cost.
As for upgradeability, both should be fine. In fact, I just installed an Avid BB7 mechanical front brake with 185mm rotor (stock was Avid BB5 w/ 160mm) today, plus Avid Speed Dial levers and SRAM Pit Stop cable housings.