I have 12x50 ELs and have been tempted to upgrade to 12 or 14x NLs.. Just hand holding 14x NLs in scheels, i was floored at how steady i could hand hold them.
12x NLs seem to be the best balance if you're going to want something you can pull off your chest and look at stuff in addition to lots of tripod time. 14x (which i think are actually nearly 15x mag in reality) sound pretty awesome for dedicated glassing open country though. From what I gather, the 14 NLs are a significant upgrade over the 15x SLCs. If you are keeping the 10x ELs, would be hard to not want to spring for 14x NLs IMO.