I've had no issues measuring a lot of canvas on the iSis. But in the end you need either both or at least the i1Pro-2. Today I had to build a profile for metal prints and no way is that going through an iSis! I once did a profile on ceramic tile that was a couple mm thick. To measure 1700 patches, three pages was about 10 minutes. For spot reading, ambient and the like, the i1Pro-2 is (was?) the swiss army knife of Spectrophotometer's with a lot of good 3rd party support (BableColor, SpectraShop etc).
[color geek mode on, so never mind about it]
Andrew - by any chance, do you know if there's any trace of practical advantage of white LED + UV LED illuminator in iSis versuss tungsten + UV LED illuminator + software M0-1-2 simulation in i1Pro2?
[color geek mode off]
If you're not scanning a lot of targets, get i1Pro2 - it's much more universal, and you can use it to calibrate, profile and measure virtually anything. Like Andrew said, it's the swiss army knife of Spectrophotometer supported by virtually anything that has anything to do with calibration, profile making or measurement. And if you're end up scanning a lot of targets, you can also get an iO Robot - it takes more space than an iSis, but does the job in comparable time.