that question introduces another question. What year sentras are we talking about? If you are comparing new SEs, with old SE-Rs, then the answer is easy. The engine is the same and the body is different. If you are talking about new SEs and new SE-Rs, then read russes review (it is very good). If you are talking about old SEs and old SE-Rs, then the difference is the engine. The SEs had the Ga16de, and the classic SE-Rs had the SR20DE (now in the new SE).
Basically, Nissan just confused the F*ck out of everyone with the switch of model change names and engines. Nissan should have named this current model the SE-R, then the current GXE should be the SE, and the current XE should have been dropped and had the GXE tag. When people ask me what model sentra I have, I tell them SE. Their next question is always: "Dosen't that have a 1.6 litre engine?"
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SR20DE..Sorry Si guys.