Does CPL look better for the promotion board than SPC?

It really doesn't matter. I was a SPC and I maxed the board. I got my CPL later and hated every minute of it. You are only a NCO when it is convenient but treated like a SPC the rest of the time. At least that was my experience with it.

If you're a CPL, you've already been identified by your chain of command as posessing the qualities of an NCO. You ARE and NCO at that point although some would tell you that you aren't a "real" NCO but if that were true, the rank wouldn't appear on every NCO chart with the creed. It sets you apart from the rest of the pool of Specialists, some of which will appear at a board after being Command List Integrated and the unit feels like they have little choice but to send them. If you have the chance, get the stripes, but BE an NCO, don't just wear the rank. CPL was absolutely my favorite rank. I don't know what other rank says above average as much as CPL in the Army.

No it is going to depend on duties assigned. CPL as Team Lead vs CPL as CO's Driver? Which do you think the board would view more favorably for leadership? So you mentioned in another post you were vehicle commander and in a E5 slot. OK, BTDT, but you know that is kind of expected of a SPC or CPL that they will assume that role in combat if the SGT gets hit right? While it will add a plus to your board submission what they are really looking for is above and beyond leadership stuff such as time as Squad Leader or Section Leader. Potentially time in other areas as well, perhaps as assistant Armorer, Training NCO, etc where you are also getting exposed to admin features of running an Infantry unit. Have you done your first counseling statement as a leader yet? Have you done EIB station training as a leader yet? Stuff like that.