They both have their pros and cons, it depends what is most important to you. For example, if flashing a custom ROM is a priority, you might want the Esteem, since the Mercury has no dev support. If battery life is a big deal, you might want the Mercury. You say you just want to compare phones, not carriers, but in my opinion, the carrier should be a big factor. Do you live in a Metro area with solid 4G coverage? If not, that would disqualify the Esteem for me (Metros 3G/1X sucks). And don't forget that Cricket only has a 1 GB data limit before they throttle you.
But back to the phones, I would say Esteem-bigger screen, 4G, hdmi out. Mercury-sleeker design, not as bulky as the Esteem, better processor out of the box, (though the Esteem has been overclocked if you do custom ROMs) better camera (on paper, anyway, probably pretty close in real life), much better battery life.
I would say they are pretty evenly matched, it just depends what you are looking for, and which carrier is better where you live.