I was wondering if I should get a mag125 or mag300 for my 90gallon with 3/8" glass. The 125 should work but I was wondering if there is advantage to the stronger magnet doing a better job or just more risk of scratching?

the risk of scratching is the same, and ti really has to do with if you get something caught between the scraper and the glass (like sand)

that said, either should be fine the 125 will be easier to work with, the 300 might get some more of the harder suck on stuff

I have personally never had any issues concerning scratching of the glass in any of my aquariums with the Mag Float. I agree with rob thought the bigger the magnet the better it is at getting the hard to scrap stuff off.

It's the arylic guys like me that worry about scratches. Glass people don't know how easy that have it. :)