I voted for the ovulation tests, simply because the first month i took them, i got my bfp with my last baby...and the monitor is expensive! But, if youve been ttc awhile, you have very irregular cycles, and you'll probably have another one, then the monitor might be a good investment
I've never used the tests but have the monitor. The only downfalls (I think) to the monitor are 1. You have to set it before CD5, otherwise you can't do it after that, so if you forget you're screwed 2. You only have a few hr testing window, so if you don't wake up in time and test, you're out for the day. It's pretty accurate, but if you forget to do it you're not able to test for that day and will end up having to get OPKs anyways.
I Only really use the monitor if I want to see if I'm O'ing or when doing treatments, otherwise I just use OPKs.