Both options are actually equivalent in that respect. It's worth
pointing out that Mnemosyne will only tell you when to review cards
which you've already learned with the software.
As for how many new words you learn per day, that's totally up to you.
Basically, stop when you're tired :-) Also, learning more than 15 new
words per day is not really recommended, as you will get a big workload
later on then.
Cheers,
Peter