As far as gtkradiant goes... you can make it a prefab. I've seen lots of large models made into prefabs. No you shouldn't have much trouble getting the model into it. The major thing you need to do is triangulate so the q3 compiler in gtkRad doesn't spit it back at you with errors. With the manner your model is built... I personally would go through it and take out all unneccesary edges. As long as it keeps the shape you will be fine. In other words ...tear it down to low poly. The castle you built could easily be built in gtkradiant.

As for which is easier... well the honest answer is. None of them are easier. People in forums claim one is better because they are better (more experienced) with that program. All of them need some finessing to get a model into game engine requirements. Of all game engines (in FPS) UT and Q3 are opposites. One is an additive (q3) 3d world and the other is subtrative (UT) 3d world. So the rules to get a model in can be different.

Best advice I can give here is... Find the game editor you want to work with and STICK WITH IT. Don't let anyone push you into another editor because they like it or say it is better. You choose the one you like. After a few months of steady practice each day you will be able to create all the game content you want. If a feature is missing or something is not working right send a bug report or feature wishlist to the creator/company. There are always new updates adding new features and fixing things.