I wasn't going to chime in on this as indeed some very sound responses have already been tabled....but...Glad you made a decision and without sounding unbiased, your best bet is likely Halifax Class (CPF was the acronym for the development project-get out of the habit of using it to describe the frigates-Yes, I know it is still used in a lot of documentation and by senior personnel). Halifax class will be around for another 15+ years whereas I suspect (my opinion only) that IRO, ATH and ALG's days are numbered. In all likelihood, from an engineering standpoint, the next generation of "blue water" vessels (SCSC?) though possibly some sort of hybrid, will likely run the venerable GE LM 2500 GT as primary propulsion.
The Control system between Iroquois and Halifax is the same but with different peripherals (equipment), it is not the same. Iroquois class technology though updated during TRUMP is 1960s with Halifax Class being late 70's/early 80s. There was most definitely quite the gap in there.
Some will say a diesel is a diesel is a diesel but for those on these forums with intimate knowledge of MWMs, Pielsticks and Allisons, I think you will agree that each has it's own separate bag of problems. (Not even mentioning the Paxman Valentas on VIC class!) I encourage you to however garner whatever experience you can-I never did that and it is too late for me. I went straight from an IRE (steamer GATINEAU) to a frigate (TORONTO) in 1998 and that was quite the culture shock but I had to ask myself once I requalled my Cert 3 "Why did I wait so long??" Deep down, to all of us, though change is good, too much is bad - You find that line and you are laughing. In all honesty, if I were starting out now, I would go Halifax class and than jump all over the AOPS (if and when....).
Anything else you have, don't be shy to ask and keep us updated on how things go.