I would try the current 20 gal you are using now, provided you like it. You can always upgrade it later.
Converting a 75 gal tank as a sump is doable, but has a lot of problems. First it's fairly tall, and second it's wide. It may not easily fit into the 125's stand, depending upon which 125 you plan on. Using a 75 gal would also be overkill. It will work, but you might want a tank like that for something else.