Hey loza, poor you-what a decision to have to face!
Who is this guy who is "interested" in your pup? It sounds a bit urgent, and it seems to be putting a lot of pressure on you to make a rushed decision. Your pup is a big responsibility, and you obviously care about him, but if he's demanding to take him tonight, I would be a bit unsure. It just adds to the guilt and so on that you're probably feeling anyway.
You are brave (and honest) to admit that your living situation probably isn't ideal for your pup-it's really hard thing to admit that. And maybe a new owner could give him more time as I am told that staffies are very demanding! I am certainly not suggesting you don't love him, or that someone else could love him more, but it might just suit him (more time and commitment) and you and your partner (less stress/guilt) better.
I am wary of the guy who wants to take him tonight though-I would ask for more time, and a more detailed background-who is he? How does your partner know him? Has he had dogs before, and/or has he done his research? Will your pup be happy to travel 4 or 5 days a week? I know some dogs that love sitting in vans with their owner, and some dogs that just hate it! It might be no more settled than his current home environment.
Also I don't know how you or your pup would feel about still seeing each other regularly-maybe a bit confusing? I am by no means an expert, maybe other more experienced ABers can comment on that.
Good luck in making your decision, take your time and don't let anyone pressurise you.
Lisa.