Getting ready to start SoU and just wondered what all you gurus out there thought about which prestige class might work best with a ranger/rogue combination? Or just be better off taking those classes farther? I don't think I get access to the HoU classes (weapon master, RDD, etc) but out of the ones I can use in SoU, which would you guys pick? How far would you take each class?

The problem here is levels, even if you push-it solo you'll likely not get past level 13 and chances are you'll end at level 11 or 12. With that in mind..

The easiest (and more interesting) class combination for SOU is the Ranger to Blackguard route. At any level in the game you won't have any real difficulties and playing an evil character in the game is fun!

VERY carefully observe the Blackguards requirements:
1. evil
2. base attack bonus = +6
3. Hide 5 skill points (not including bonuses from your dexterity attribute)
4. Cleave feat

If I were you I'd get the Hide and the Cleave feat out of the way initially - and perhaps the evil requirement as well (BUT you can easily change your alingment to evil several ways in the game). This then allows you to NOT worry about meeting the Blackguard requirements. Then just pick Blackguard as early as you can and stick with it.

btw, there are fairly nice perks for both Rangers (animal companions & tracking) and Blackguards (I'll leave that one to you).