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.