I think it's a take/take situation. With Minnemann, the band would probably have kept that 'sweaty spark' of fun and improvisation that was present with Portnoy, and I'm not quite sure if they would have played with a click track with him. With Mangini, the band turned into a more 'academic' side of music and it shows. Both approaches aren't bad, they are just different.
For the record, Minnemann is my favorite drummer. I would have loved seeing with with DT, but as it has been said for the past 5 years, Mangini is a perfect fit. Taking the bigger picture into account (OMG NUGGIT), keeping Mangini was the best decision.
Also, if Minnemann had gotten the gig back then, we probably wouldn't have had the glorious Steven Wilson band he have today.