What year spec-V?  If its the new one for 07, I'd go for that.  They are bigger than before and make 200hp n/a.  They stopped making the Altima SE-R, so you must be looking at a slightly used one...  Best bet is to test drive them and see what you like.  The Spec-V is your best bet for the money if you are getting new.  If you are looking at a used car get the Altima SE-R, but I would think it'll still cost more than the Sentra.  Go drive the Spec-V tho, I think you will really like it, its bigger than other cars in its class and has some trick options.