Two payments: one on the 15th, and one on the 1st. This can put some people in a bind, especially if you run things very close to the wire.
2checkout also has lately (in the past year) had a few incidents where their network was DDoS'ed. However, since they target the online services industry and thus have a relatively high visibility on the script-kiddies' radar, it's kind of inevitable that they'd get virtual rotten tomatoes thrown at them.
Until you can get a merchant account, you pretty much pick which set of problems you can live with, and then go with that payment processor. Paysystems and 2checkout were about equal (except for Paysystems' weekly payment schedule), but then they pulled their little "we're going to make you(r clients) donate to our charity" stunt...and I scratched them off my list.