Mk8 is blocky but dehorned and pleasant. Very solid mounting. Antirotation ears snug up to the receiver with setscrews - very precise but probably unnecessary.
MI I have only had the G3s and some related models; they appear largely the same as your SP series. Very smooth, not blocky. Antirotation isn't as finely tuneable as the Mk8, but I don't think it matters.
MI bang-for-the-buck is somewhat better, but the Mk8s can be had used for a very reasonable price.
Specs for both of the 7" rails are right at 9 oz or so.
I guess it comes down.. both do the job very well, but the Geissele is bigger, blockier, "nicer", and more expensive.