Related:

https://forum.colemak.com/viewtopic.php?id=559
https://forum.colemak.com/viewtopic.php?id=1585

Colemak and Dvorak are leaning to the right hand (Colemak 6%, Dvorak 14%), so I suggest using the left thumb to press Space bar to balance the workload between hands.

On Dvorak the left hand remains relatively stationary so left thumb may be better. The thumb you use is more important on a touch screen if you're thumb typing, where you want to use the lesser used thumb for space.

According to this article http://gizmodo.com/5944694/is-the-extra … or-useless
"Internal research at Microsoft shows that more than 90 percent of people hit the spacebar with their right thumb"

I wonder if that's really true though.  Certainly I use the left thumb despite being right-handed. I realize I am statistically insignificant though :)

I am interested to know which hand others use, especially if you use a non-standard setup, such as the Wide Mod or remapped AltGr.  Does anyone (with a standard keyboard) use both thumbs for different functions?

I'm left handed, and I always use the left thumb to hit space.

Although, I wonder how accurate self-testing is, and I may very well be hitting it with the right-thumb sometimes.

Just wonder...since you also use the wide mod, why don't you always use left thumb for space, leaving right free for AltGr?

This is an old post, but no one has mentioned the frequency of letters at the end and beginning of words. The most common endings in English are e, s, d, and t. The most common beginnings are  t, a, o, d, and w.

I mean, this is an oversimplification because we have a lot of really common short words like "the, and, to, not, has, was, we, he it, that", etc. that should be taken into account due to frequency. But I think it's still pretty relevant.

So if you are on Colemak, only the "e" is on the right hand for word endings. For word beginnings, you only have "o" and "d". The difference between the last letter of a word and the  first letter of the next word isn't going to matter in each individual case unless both of those letters are on the same hand. A much easier generalization is to think of which hand gets used most on the beginning and ends of words. For Colemak it seems like using right all the time would be more efficient, if it made a difference.

On the other hand, with Dvorak you have the "e" on the left hand but all the others (s, d, t) are on the right. With beginnings, there are still more consonants on the right. I don't know the frequency of word beginnings and endings of vowels vs consonants, but maybe using your left could be more efficient on Dvorak.

Anyway, this is an oversimplification and it probably doesn't matter enough to cause a difference in typing speed or comfort, but I don't think it would matter if you're trying to alternate on the word before or word after, as the OP was asking. Rather I think you have to look at the bigger picture.