I had a 3.0si and an M at the same time. I enjoyed them both very much. I did finally sell the 3.0si but I honestly still miss it.
For a daily driver I'd keep the 3.0i. Upgrade it to a 3.0si with 3 stage intake manifold + associated changes to the tune; this is very popular, well documented, and won't affect reliability. It *will* give you 50 extra horsepower.
And consider a limited slip differential too. That's what I would have done with my 3.0si if I had kept it.