Is there a difference besides line capacity, between these two sized reels, is one better than the other, or is it just size and line capacity?

Just size and line capacity, its a personal preference choice. Some may try to say the 50 size reels cannot handle some heavier applications but its not true. You can punch, frog, jig, etc with a 50 size reel with no issue.

I have 50 sized reels on my jig rods because they are a little lighter.   I routinely use 15 and 20 lb Abrazx with few issues.

yes..it really depends on how often you will use them. The 50 is lighter and easier to palm, but can handle almost anything except for maybe larger swim baits.

Also the 50 sized reels will cast lighter baits down to say say 1/8 or so better. I also like the fact they are lighter and easier to palm.

Internally they are identical. The shallower spool on the 50 makes them capable of throwing lighter baits. Outside of that they're interchangeable as far as application 

50/51 are great for lighter bait, down to 1/8oz 

I like the smaller profile of the 50 and 70 sized reels for most every bass fishing application aside from bombing heavy crankbaits or swimbaits, in which a larger diameter spool would be an advantage.