I was stashed as an assistant recruiting officer at an NROTC unit. Feel free to PM me with any questions about NROTC regs.
While I don't have the actual stats, I do believe it would be 'easier' to receive a commission through NROTC. Here's why...There are several different scholarships offered that would have you ending up at an NROTC school. If you have three years left, you can easily still commission under NROTC.
If, for whatever reason, you don't receive a scholarship (and essentially a commission) you can still commission under NROTC through advanced standing. Basically, the CO, OD (Officer Development) and other higher ups agree that while they didn't give you a scholarship you still deserve to commission. It doesn't happen a ton because usually people pursue other ways to commission. However, if you stick it out, keep you PT and grades up, and generally give a shit, there is a good chance you will earn a commission--also having a technical major helps a lot.
If you would like a comprehensive understanding of how NROTC works, download the 'ROD' (regulations for officer development). Its a huge document that has a lot of stuff you don't need to worry about--but the hyperlinked TOC will take to you to whatever you need to know.
I would imagine he is correct in saying NROTC receives preffered selection simply because of the 'training' Midshipmen undergo while in the program. However, understand that NROTC only commissions unrestricted line officers (SWO/SWO(N), Subs, SNA/SNFO, EOD, SEAL)--with a few opportunities for Docs, Reactor engineers and EDO. You didn't specifiy if you wanted aviation, so just understand with NROTC, you won't have a chance to go Intel or PAO unless you really mess yourself up medically.