About the same in performance but, as mentioned, teeth are everything! If one has sharp teeth and the other dull, you'll think the one with sharp teeth runs great and the one with dull teeth is a pile of crap. Switch the teeth around and you'll say the opposite for each machine. that's the problem with rental machines - they usually run the cheapest teeth and don't change them until they break off. You, as the renter, end up spending 3 times as long as you should grinding a stump because you end up beating it to oblivion with dull teeth rather than actually grinding it. That's why I only rented for one year then bought my own 252 which I then put greenteeth on.
As for the 1625, also owned one of those with greenteeth. Worked fine though i did like the 252 a little better so sold the 1625 but, like I said, pretty similar in performance if you have good teeth on them.
AP