I would say it is all down to personal preference. I have not done that much racing myself yet, but at the moment I feel more comfortable in an all in one suit. I think it also depends on the length of the races you will be doing.
I would say for the shorter race you would just need to go for whatever will save you the most amount of time in transition, this is where I think the all in one suit would prove beneficial. If you are doing longer races you would have some more time to put on different layers depending on comfort - IM you wouldn't want to be in an uncomfortable suit fo 180km on the bike!
So end of the day, have a go in training with different configurations. Have some races as a 'C' race (scale of A - C, A being you goal race, B being the nice to dos and C being a training race) try out different kit in your C races too so when it comes to your A races you have the right kit for you.