It really depends on your driving style. The SSEi is better if you like the extra power when needed and it'* a more reliable powertrain. The SLE is quicker around town EXCEPT for if the SSEi hits boost. Once the SSEi sees boost levels it is significantly faster. WIthout hitting boost the SLE is slightly more peppy from stops. (because of gear ratios mostly.) The SSEi we have averages around 23.1 MPG average and our SLE averages 25.4 average. (haven't reset the gas mileage readings in about a year or so.)
We own both a 00 SLE and 00 SSEi by the way.. I've driven both side by side to come to these conclusions. Myself, I think the slightly less gas mileage is worth the extra horsepower when you tip into the throttle a little bit.
My only complaint is that the SSEi really is a slouch until you hit boost.