It's a really tough choice, because they're both great for different reasons.
Great Bear has that really neat helix on the first drop which is the thing that catches my eye most about it. Also, I've heard it's fairly intense, but nothing too intense. However, I'm sure the fun makes up for the lack of intensity in some spots. It's one of the most visually appealing coasters though.
Nemesis looks great. First of all it has that wicked themeing! It might look pretty small but I have a feeling it's one of those small but extremely intense things. The final helix I heard is great as well.
Overall, if I got the chance I would probably ride Nemesis, but they both look amazing.