i would think that a crib, designed for sleeping is much better for just that...sleeping.
a playpen is designed to keep a baby/toddler safe for short periods of time. they can be used for sleeping, but i would only suggest it for short periods of time, or in emergencies, like you are on a holiday and that is all that is available.
i would get the crib first. then after a few months, the playpen. it isn't really necessary. i found that we ended up using ours as a toybox because my son HATED being in it. he much prefered the floor. a young baby doesn't become mobile until after 6 months, so it is perfectly safe to put the baby on the floor to play. once they are crawling, then you can still let them play on the floor but you need to be much more cautious.