You probably won't notice much difference in battery life when switching ROMs, as most ROMs use the stock HTC kernel by default. You'll probably notice a difference when trying a different kernel. That being said, when you flash a ROM, try it out for a while on the stock kernel - maybe a couple of charge cycles. Then consider switching kernels. Netarchy is a good choice, and will likely result in better battery life. Just remember that there are other factors that contribute to battery life and drain. A different ROM or kernel won't make a difference if you use your phone very heavily or simply do things that drain the battery.