It really depends on the trade. Not all trades that are "In Demand" are at the same level. Also, just because it's listed as "In Demand" this week, it doesn't mean it'll be that way next week. It could also already have a lot of strong applicants, and a plethora of other factors.
Don't pick an occupation that's "In Demand" because you think it'll get you in faster. It may not go any faster than another occupation that isn't "In Demand", and you could end up in an occupation you don't like. Your recruiter may tell you that you can transfer later; while that's true, you need to be a Corporal before you are eligible (3-4 years after you get in) and you'll have to go through another competitive process to be selected.
Go for trades you actually want to do, don't worry about whether it's In Demand. A few extra months of waiting now could save you years of trying to switch later.
My :2c: