You can get a rough idea of Amazon e-book sales from different publishers at salesrankexpress.com - it might be a good idea to type in the two publishers and see how their books tend to do. You could also look at the last few releases from each publisher to see how they've done.
Overall, I don't think the e-imprints of the Big 5 are doing as well as one might think they would do. At the same time, they're almost certainly more stable than most e-pubs.
There are a handful of e-publishers that I would work with in preference to an e-only imprint from the Big 5, at least right now. Samhain for sure, and for me, writing m/m, Dreamspinner and probably Riptide. I haven't tried all the other e-pubs, obviously, but I think I'd try the Big 5 imprint over any other e-pub.
But it's not a clear choice, to my mind.
ETA: Maybe add Entangled to the list of e-pubs I like better than Big 5 e-imprints. I've enjoyed working with them, but haven't seen sales results yet. And they only work if you're writing very trope-driven books, I think.