JP1 Remotes

I have an OFA 8820, and although it is not as easy to use as my favorite 15-2116 (which ran out of memory ;-( ) it is usable, and I got it to do most of what I want without any JP1 programming. However, I would love to have a few more programmable keys, and the 10820 has 3 more "learning" keys (plus 2 more devices, which i do not really need).
Do you experts have any opinion about the relative merits of the two? The manual is a bit murky on the learning side of the 10820, suggesting that only the 3 L keys can be taught, while in the 820 most keys can be taught new tricks. The 10820 also uses 4 AAA batteries to the 820 2 batteries, which I like.
Your views will be appreciated.

The URC-8820 and URC-10820 (and the 6820) are functionally identical remotes. The 10820 has a few more physical buttons (which are available on the 8820 as phantom buttons).

the 10820 is about .5 inches wider at the top and .5 inches longer than the 8820 which allows the keys to be spread out a bit. I usually prefer the smaller remotes, but I like the 10820 much better than the 8820 simply because of the key spacing.

What are phantom keys, and how do I use them?
The main reason I am asking about the 10820 is the extra keys that it has, compared to the 8820.

Thanks-- that document is great.
Isn't it odd that just for adding 2 extra devices and 3 learning keys the 10820 needs 4 AAA batteries to the 8820's 2?

The extra devices and keys don't create any need for extra power.

I'm not sure which advantage of 4 batteries over 2 was significant to the decision. I can think of several possible advantages, even if 2 batteries are enough to work.

The obvious disadvantage of 4 batteries is size and weight. One might reasonably conclude that customers who really care about size and weight weren't going to select the 10820 anyway, so battery size and weight mattered less in its design than in other models.