For a single night it's a toss-up.
The rooms at the Grosvenor are at least a decade behind in renovation yet are (usually) clean. It should be tolerable for a single night. Seralgo's rooms are much more moderized but are simply basic motel accomodations (in other words, very little advantage for a single night stay). Neither hotel has a spactacular pool and both have advantageous locations (Grosvenor = walking distance to Downtown Disney & Seralgo = inexpensive resturants and Old Town).
You didn't list the rates, but chances are the Grosvenor will be significantly more. Don't forget about their B.S. $9 nightly resort fee. Truthfully using all resources you can get a better deal elsewhere (than these hotels). But what date are you looking at? And what do you desire (just a place to stay, a good location, a good pool, etc.)?