if you have your thoughts on the s/c, i would do it. the 4cyl will make some decent power and the price isnt too bad, considering our friken dollar is worth NOTHING right now..
but as for the 6cyl. the weight will be more, and you need to get everything such as rad, ac compressor, downpipes/exhaust, front swaybar from a 6cyl model. so it can be more work than putting a s/c on. plus i like the whine the s/c makes on the 4cyl..