Go with your gut. Since your not comfortable then go for the trimmed, especially if it's your first time. You'll still learn a lot about trimming while building it. AP does an excellent job with their trimmed armor but you'll still want to try pieces on before gluing them together so you may still have to do a tiny bit of trimming. You just won't have to deal with all the excess abs material.
For untrimmed, as pattrooper said, take your time if you choose to go that route.
Either way, always have a sharp blade in your knife