Comparing a 26" MUni to a 20" freestyle Uni:
The wheel on the 26" MUni has greater inertia, while the lighter wheel of the freestyle uni has less inertia.
The tread on the 26" MUni has large surface area touching the pavement, giving it a lot of grip, which creates resistance to turning, while the freestyle tire is under high pressure, has a small surface area touching the pavement, and is less resistant to turning.
So, which is better for a beginner?
The freestyle unicycle provides better feedback about balance, and will behave in a more twitchy fashion. You will give laypeople the impression that you are just learning, because the smaller unicycle will be rapidly shifting left and right.
Conversely, the larger unicycle will tend to auto-steer itself, masking some feedback about balance. However, you may use the momentum of the larger unicycle to get in a few revolutions, which is important for the beginner’s self-esteem. You will appear more poised on the larger MUni, though you may not be sure what the heck is going on.
A few neighbors commented to me that I had improved when they saw me riding a larger unicycle. Don’t be fooled: just because you ‘look’ balanced, that doesn’t imply your balance has improved.
Either way, I hope that you, at some point, can experience the joys of a 20" unicycle. If you buy a 20" trials/street unicycle now, you will be able to take that on the trails, no problem. Learning to idle, ride backwards, jump, seat-in-front…are all fun, and generally easier to learn on a smaller unicycle.
I think it’s pointless to worry about, as a beginner, the relative slowness of a 20" unicycle. It will take time for you to develop cadence on whatever unicycle you are riding. If you get a 26" MUni, you may be riding pretty slowly for the time being.
You’ve heard about medical conditions in which the patient sees double. I think you are seeing ‘single’ (perceiving that ‘one’ unicycle can meet all your needs).
BTW my MUni is an Oracle 26". I think the Duro tire is overkill for most of the riding I do, and I’m looking for improvement in the hill climbing department, so I’m downgrading the size of the tire. Also, I had a KH t-bar installed and am practicing riding flat/uphill with both hands on the bar ends. If you are thinking your 26" MUni could be your all-in-one unicycle, I’d recommend the t-bar, installed close to the seat; it will take time to learn how to use, but the benefits are huge.
Good luck learning to ride.