I went through the same pros and cons. I personally think the key here is "moderation". Don't overheat yourself, or over-exhaust yourself at all. Exercise until you get to the point where you just BEGIN to start feeling yourself slowing down. And keep it at a comfortable pace.....you don't necessarily need to speed. Don't do like other exercise instructions tell you...........to go past that point until you find your "zone" or get your so-called "second wind". That won't happen in MS. Once you BEGIN to feel even a tiny bit of struggle..............stop right then. I don't care if it's only walking for one minute the first time. Stop. Then the next time you try, you may find that you can go two minutes. Of course, I'm no expert (I say that a lot, don't I? LOL That's because I'm so opinionated, that I don't want anyone to think I actually know for SURE what I'm talking about! LOL)
Sometimes just resting for 15 minutes or so and then trying again is supposed to help. But you know what I did? I thought....to heck with that. If I can only go one minute, then that's it. Now I'm up to 20 minutes (I do stationary bike, which also has the thingys that exercise my arms, too), and I have found that that's all I can do. 20 minutes is my top. But.....I add stretching, and a little weight lifting with 2 pound weights (yea, I'm a wimp...LOL), and sometimes when I'm feeling spunky, I'll dance around the house to music. (With the blinds closed.) LOL And of course, I walk some. Especially at work!
I'm not sure what your current physical condition is right now, of course, but I know what you mean about the expense of a treadmill. Plus, if you have balance issues at all from time to time, a treadmill may end up not being the best thing (that's why I opted for the stationary bike. I figured I could "walk" around the house on my own, if I had to. Bumping into walls, but still........LOL) That was all last year, though. This year, I'm back to doing a little "jog" now and then, not to mention a little "jig", too.
(Uh oh........I'm in a "mood" today, huh?)
Seriously, though. I believe that moderate exercise is highly beneficial in MS. And yes, you do still have to factor in your energy bank.
You know what else I've read many times, though, and what I myself found was the best to do? Exercise in the evening. For people like us with a finite amount of energy, it's the best time since we don't have to worry about "overdoing" thereby compromising our abilities for the rest of the day, AND contrary to the rest of the world, it actually HELPS us to rest and sleep at night if we exercise in the evening.
Well, I hope this helps in some small way.