So the price difference for new pedals isn't really that much different between the two, but I am looking for a used EQ because I eventually want to retube my amp, so I'm pinching some pennies.

Here is my question: Is the difference between the MXR 10 Band EQ worth the price difference between a used Boss EQ? If I go with the MXR, I will likely be buying it new because the used market is a little scarce in my area. On the other hand, if I get the Boss EQ, there are so many used ones that I can score one for about $50. Is the MXR that much different than the Boss to justify that much of a price gap?

I will be using the EQ in the effects loop of my Valveking 100 head, and my tone is similar to Mastodon, and at times maybe a little Gojira-ish.
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I've owned both (actually have two MXR 10-bands at the moment). To be fair, I owned a Boss GE-7 waaay back when I was a teenager and didn't know what the hell I was doing. Both are quality pedals; the Boss is a little noisier (not an issue if you run a noise gate though). The MXRs don't add any perceptible noise when used in the loop, in my experience.

Other than one having 7 bands and the other having 10 -- if you look at the frequency ranges, their sliders are at slightly different ranges. Some people say they like the Boss's selected frequencies better -- others like the MXR better.

One comment I hear from time to time about the MXR is that "you can't hear a difference" on the highest and lowest sliders. Well, that's mostly untrue. The 32 hz (lowest) slider is mostly unused, because guitars don't produce frequencies that low. However, I tend to cut that one anyway because I don't want any energy expended producing frequencies that people won't hear/the guitar isn't producing naturally. I don't think it truly makes a difference though -- it's just a habit. On the high end, however, I can hear a BIG difference when adjusting the 16 khz slider. Namely, if you set it for a "cut" to that frequency, it will tame the obnoxious fizz that some amps have. Depending on what amp you're running (or speakers, for that matter) it can vastly improve your tone. So anyway -- at least 9 of the 10 bands are quite useful.

As a side note, I am planning on selling one of my 10-bands. Some of its sliders don't light up, but all frequencies work exactly as designed. A/Bing it with my other one and adjusted the same, they sound identical. If you're in the U.S. and interested, pm me and I could cut you a deal.
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I use my MXR 10-band kinda like a tubescreamer, as a gain boost in front of the amp. In general I feel that the extra three EQ bands the MXR has over the Boss is an advantage. Worth the extra coin? That's up to you, but it is to me. Good luck!
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I've had both a 7 band MXR and I currently have a BOSS GE7, Personally prefer the BOSS, Ironicall to the above post I found the MXR to be noisy and not the BOSS one, I like that the BOSS has a db boost slider on the end too, or a mix, I forget what it is exactly but I like it, the MXR doesn't have that. If you can try both out I'd recommend doing so, personally though I recommend the BOSS
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I've always preferred the extra frequency controls with the 10 band. Used deals can be found online and shipped if your area is dry to make the price a little more comparable for you.