Having gone through what your thinking about with a car that was in the condition of yours (and is in fact the same color) with very like miles on it, I'd do what the others recommend. In other words if it was a year ago I'd just keep the B18 for at least another couple of 100K and work on the non drive train stuff.
The street performance B20 certainly has more pep...actually the thing roars...and makes more noise, but I'm not so used to it to make an informed judgement at this point. But in a way I miss the frugality and reliability of the old B18. I suppose I could stuff the thing back in there if I wanted. However...