there are pros and cons to both. Semi-dry is similiar to a wet suit and while I have had them, Mares isotherm, and enjoyed it I am now using a dry suit. The Semi-dry is lower in cost and has many of the same things a dry suit has, zipper, roll cuffs to keep out the water but in my case the boots were still wet. Weighting is the same as a wet suit and there is no valves to contend with. That said some of these are the cons as well. As it compresses it looses thermal protection that dry suit undergarments and the air/argon can provide. Dry suits have dry feet and may require a bit extra weight than you would normally use on a 7mm wet suit depending on your undergar selection. Extended underwater time syrely the dry suit has benifits as it protects you better. They also require some extra caution, seals can rip, if they are punctured or cut the comfort of the dive is over and depending on the type of dry suit the thermal protection is reduced or eliminated all together.
My Suggestion would be to take a dry suit class or try one during the DUI dog days when they are having an event close to you. We have one here in October. While you cannot purchase a dry suit then you can try on many different types, dive them with an instructor escort and even get your dry suit speciality course if you so choose. Dry suits while they do cost a bit more than the semi dry can provide years of diving pleasure in colder waters allowing you to extend your diving season and see things you normally do not during the warm water season.
Hope this helps and if i can assist further dont hesitate to contact me.