Do you have exactly those two cards, or just cards with the same chipsets? If not those cards which ones do you have?
And do you intend to use them under DOS or Windows?
ESS' chipsets are some of the least troublesome SBPro2-compatible cards for DOS, and don't require a TSR, which means no loss of memory. The ESS FM synth isn't exactly OPL3, but close enough unless you're a real purist. For DOS only, this is probably the best solution. It will work fine in Windows too, as SBPro2.
OPTi 92x chips have decent but not perfect SBPro2 support under DOS, and do actually need a driver to run, but have WSS support as added feature, which isn't really great in DOS (few games use it), but can help in Windows.
Both chipsets have been used on very good cards and on extremely crap ones, so the cards you have can decide the question more than the chips. Some also came with wavetable support on-card. I have an OPTi-based one with (pretty crap) wavetable. Not exactly going to beat Yamaha or Roland anytime soon, but better than nothing if you don't have sound modules or high-end cards.
I don't really see a reason to use both, they have big overlap in terms of features.
WIth a P120 I really wouldn't recommend the SBLive or any other PCI card for that matter. Given the SBLive is notoriously PCI-sensitive (think Via 686B-bug), I wouldn't even be confident it would work at all. It's a great Win98SE card (just use .VXD drivers, not .WDM), but in DOS it needs TSRs like almost all PCI cards. Only upshot is that it supports SB16 (but not SBPro2). You could try it next to one of the other two if you can get the resources to play nice to compare SB16 to SBPro2...