I have both of those coils. In my soil here is what I've found:
The 10.5" HF is a magnet for dimes and quarters. Surprisingly it does pretty decent with nickels. Pennies are no problem. For the longest time that was my favorite 10.5" coil. For about the past year or so I've been letting my Mom use that because when she first got her detector I didn't think she was doing as well as she should have been doing with the gold pack coil. It's just to easy to find coins with the HF coil but the depth isn't quite as good as the MF coil. I would say that the HF coil is very forgiving because you can give it a pretty fast swing speed and it will still sound off very well.
As far as the 10.5" MF coil goes......for the longest time I didn't like that coil. Like the HF, it's kind of a magnet for dimes and quarters but I never seemed to find anywhere near what I did with the 10.5" HF coil. If I went over a site with the HF coil and went back with the MF coil, I would hardly find any additional coins. If I went over a site with the MF coil and then went back with the HF coil, I would typically have a significant number of finds.
After loaning my Mom the HF coil I kind of forced myself to use the MF coil even though I had a ton of hours on it already. The first thing that I'll say is that it has a tough time with nickels. You'll find them but I've always felt like between the two, the HF will find nickels about 6:1 when comparing the two. Obviously that's just a guess because you never know how many nickels either coil has swiped over but I say it's a good guess.
What I eventually found out after many more hours of detecting with the MF coil was that you really have to go much slower with it compared to the HF coil. I typically detect at a reasonable speed giving the coil and detector time to work but after I slowed down even more with the MF coil it started to shine. I've found that I find deeper and older finds with the MF coil compared to the HF coil. I think if you are using the MF at it's optimum speed, it can be a little prohibitive in covering a decent amount of ground unlike the HF coil. I still call the HF coil "The Sweeper" because you'll pretty much just sweep coins up with it in the right environment. The MF coil in my soil can easily keep up with the HF coil but it seems like it just needs a little more time to find it's targets.
I have to admit that since the two of us have gotten the 6" Coiltek coils, that coil has pretty much been a fixture on our 705's. I always say that "if this was a professional sport, the 6" Coiltek coil would be Illegal". Once you use that coil, nothing else really compares.
If I'm at a site where I need to cover a lot of ground I'll put the 10.5" coil back on and then after I've dug all of the "gimmies", I'll swap it out for the 6" Coiltek coil. If it's not a huge site then I stay with the 6".
So, to answer you question.......your ground may be different than mine so our results may be different. If you ever salt water beach detect the HF may be the better choice. If you don't mind going pretty slow then you may find older deeper targets with the MF.
I don't believe that it's worth having both of those coils. If I had it to do over again, knowing what I know now, I'd probably just buy the HF coil. That decision is mainly based on the fact that I typically never had many additional finds after going back to a site that I went over first with the HF coil and then used the MF coil. Finds were significant in opposite scenario. I don't believe that slowing down with the MF coil has caused me to find more coins compared to what I would have found with the HF coil but targets are more distinct and easier to find at the slower swing speed. Again, I'm almost shocked when I find a nickel with the MF coil. You'll find them but their sound and ID are nowhere near what you get from the HF coil.
