Have a look at the presentation to the FDA's VRBPAC on 2021-Oct-15:

K Lyke, et al., "DMID 21-0012 - Heterologous Platform Boost Study Mix and Match", FDA VRBPAC 2021-Oct-15 Materials, retrieved 2021-Oct-15.

(If you're curious about the how the whole meeting went down, I wrote a little summary.)

Slide 22 (page 23, because the FDA tacks on a header page) should help. It's a 3x3 array of the 3 original vaccines x 3 possible booster vaccines. It shows the factor by which antibody levels are boosted by each primer/booster combination (geometric mean titer fold induction).

For your case of a Moderna primer, the middle column is the one to look at: Moderna booster gave 10x boost, JnJ booster gave 6.2x, and Pfizer booster gave 11x. Given the error bars on that slide, Moderna and Pfizer are more or less indistinguishable (10x vs 11x), but both are likely better than JnJ.

In terms of absolute ab levels (the number shown in blue), a Moderna booster is probably a touch higher than Pfizer (3727 vs 2801), though I haven't used the error bars to calculate the statistical significance of that. Both are pretty good.

For me, the bottom line is that it looks like either Pfizer or Moderna are fine for somebody who's already gotten 2 doses of Moderna. The important thing is to get the booster if you need it; the choice of Pfizer or Moderna is over-optimization.

Excellent question, and I think a lot of us are wishing we had more data on this—unfortunately, there is very little data so far. But here's my take:

1. If you had J & J for your first shot, I think there's enough evidence now to say it's probably (p = 0.7?) better to get Pfizer or Moderna for your booster.
2. If you had Pfizer / Moderna for your first two shots, my instinct is that J & J might be the better choice, because there's an argument from microbiology that mixing types might produce a more robust response.
3. If you had Pfizer / Moderna and want an mRNA shot for your booster, I don't think it'll make much difference which one you get: they're very similar.

There's an argument to be made that absent strong reasons to do otherwise, it's best to follow standard practice (in this case, to get the same brand of booster as the original shots) simply because you'll be in a larger, better-studied cohort.

A couple of sources, such as they are:

“But something has really become clear: The mixing really is most impactful when you have a DNA/adenovirus vaccine first followed by the mRNA vaccine,” Gandhi said. WaPo

The study’s researchers warned against using the findings to conclude that any one combination of vaccines was better. The study “was not powered or designed to compare between groups,” said Dr. Kirsten E. Lyke, a professor at the University of Maryland School of Medicine, who presented the data. NYTimes