I'm about to do Intermediate Latin next year - but I did Latin in VCE, so I didn't do Latin A and Latin B. People have told me they enjoy Latin A and Latin B if you're starting from scratch, but I think it is quite demanding. I've looked at the grammar they do, and it is basically the amount I learnt in 4 years (granted, covered quite slowly in school) in the space of 24 weeks (with 3 lectures a week if you go for the whole year - you do have several breaks to catch up though). You do that much in 6 weeks if you do the summer intensive!! However, I've been told it's marked reasonably fairly, it's a small class so everything's marked by the lecturer, she's very accessible. If you're good with very structured subjects, I suspect you'll be good at Latin because although there is some rote learning, it becomes very structured after that - for a language, barely subjective at all. Probably quite manageable if you made sure to spend an hour or two reinforcing few nights.
I've heard lots of good things about Spanish too though - apparently quite easy to pick up. Grade-wise I think quite reasonable. Sorry I don't know more about that.
In short, both are meant to be quite enjoyable, not exceptionally easy (there are easier breadths out there, admittedly), grades not bad (from my soon-to-be lecturer in Intermediate Latin, you hand in weekly sheets, but he only takes the best 8 marks for your final mark (for 12 weeks)). Might not help you pick between them, but both sound like winners to me!