Will you be doing pans and tilts? Are you going to be in the business very long?
If your answers to those two questions are "yes" and if you want to spend as little money as possible, the 503 is your only answer.
The 501 head is awfully close, but isn't quite as smooth at the start and end of pans/tilts. It might be my choice if I wasn't all that serious about videography.
If you're not ever going to pan or tilt, then it really doesn't matter as much.
When selecting your legs, for events, look for quick setup/take down, sturdiness, enough weight to not lift a leg when panning or tilting, quick leveling (ball leveller is often a good add-on), visible bubble level and adequate height. Also, if you'll be working outdoors as well as in, look for feet that can switch from rubber to spikes, quickly. So many tripod reviews overlook height. We shoot a lot of weddings and it's been our luck to work with photographers who are not only agressive but tall. The ability to raise my camera seven and a half feet, lets me shoot over everyone's heads when I need to.
I don't necessarily like the high angle, but when there's two photographers in front of the cake, so close that icing is getting onto their dirty tee shirts (can you tell I "love" wedding photographers?) there is no other choice for this "must have" shot.
We cover this topic much deeper in our video course.
Remember - you'll likely go through several cameras during your video career, but make a good first tripod/head choice and it will stay with you for the duration!