I had 2 turbo-diesel 4 cylinder BMWs before I bought my 1st Coupe, and driving that with a 3 litre N52 engine was a true enlightenment! A 3 litre 6 that revs to 7,000 rpm - what's not to like? What had I been thinking?
So much so that soon after I bought an E46 with the M54 engine as a daily, which was good but nothing like as special as the N52 in my Z4. And that also needed a full cooling system refresh due to a bowed radiator that started to leak. :roll:
In fact I like them so much I'm now on numbers 3 and 4 with N52 power! :worshipbmw:
As others have said if a 3.0Si is in budget that's what I'd go for, but if not I don't think a 3.0i would be a disappointment.
Rather than worry about which engine is better I'd be more concerned about how and why both lower rear suspension arms got so badly damaged they needed replacing at MOT a couple of years ago - rear end shunt maybe?