In my opinion, going from a 90 to a 125 is only a modest step up in size. That's ok, but don't restrict yourself because of the sump you already have. In the long run, you'll likely replace all that anyway. So if you really want a larger tank than a 125, and can afford the additional costs, go for the larger tank. In the grand scheme of things, the price of a new sump will not add that much to the total cost of the system.