you will grow out of a correct sized semi-dry much faster than you will a over sized dry suit. For the semi-dry to work it still has to function on the same principles as a wetsuit. It has to be a somewhat close fit to work properly. The big advantage of the semi-dry is the seals and entry, you flood it once when the dive begins and even with a full range of motions there shouldn't be an intrusion of new cold water, allowing you to have to do less work and not heat up so much water with your body.
In cold water diving I lump semi-dry and wetsuits in the same category, if you want any real sort of thermal protection and comfort in water below 50 you go dry or go home.
If you buy a semi-dry one or two sizes too large, you will have a lot of extra water to be heating, and the seals will be more likely to allow flow with movement as few semi-drys, not even the mares isotherm have true dry suit seals, making nothing more than an over priced wetsuit.
drysuits on the other hand usually come with overly small necks and cuffs, to be trimmed to size by the diver, so they can grow with you. If right now you would take, say a mens medium, you can get a large. you can always use a crotch strap to bring up the waist, and a little duct tape or gators to tighten up the ankles, with your bc bringing in the chest. A drysuit just gives you a bit more wiggle room when it comes to growth, since a little extra space can be adjusted out, and is even needed for extra clothes.
As for types, I would suggest a sea elite neoprene drysuit, they are $600 brand new, and are sturdy enough to stand up to commercial work. A good entry lvl suit, easy to repair, and a bit more forgiving than a tri-lam or vulcanized rubber, when it comes to run ins with mussels and other sharp objects and critters. Also doesn't require much in the way of undergarments.
also drysuits tend to be a bit easier to sell on the second hand market, as they tend to receive much better care than wetsuits or semi-drys. I buy 3-4 new wetsuits every year, as I tend to tear them up working, but I still have the same 2 drysuits as when I started working.
Of course most importantly, if you do go with a drysuit, make sure you get properly trained, they are a whole different animal, in terms of care and use.