Odenton is the busiest station on the Penn line, from what I've read - using it daily, I'll certainly not dispute the fact.
There are 3 lots at Odenton, 2 on the SB side of the tracks, 1 on the southbound side, connected by a tunnel under the tracks. The NB lot is the smallest, it fills up with folks headed for the 6:24 train (this is the lot I use, as it's closer to my house). Of the two lots on the SB side, the closer one reportedly fills up by 7:30 or 8, and the farther one (still a short walk) pretty much always has spaces available.
I would HIGHLY recommend planing on using Odenton, as that's the only station that you mentioned that's on the Penn line - many, many more trains, more often. It gives you a lot more flexibility on when to go to and from work, and many more options to leave early/stay late. The Penn line also has mostly the Kawasaki bi-level cars, which are the nicest in the fleet. I've driven to Savage a handful of times and the commute is nice, the Penn Line is just nicer.
Hope this helps!
Ryan