So I can give my input here a little, I was just in the same position as you for the most part. (I do own an offset already).
I recently relocated from Houston to Denver and I had the pleasure of learning to fight the cold. I am a weekend BBQ'er and we enjoy hosting friends over. I usually cook for 2-20 people or somewhere in between.
With that little background I think it all depends on your needs. As previously mentioned I have an offset and I love it, but for cold temps you really need an insulated box or at least a 1/2" in my opinion. You also get the joy of tending the fire more and being heavily involved in the cooking process. I would not want to fight ambient temp plus fight internal temp while cooking, I did it once and I didn't enjoy myself. Another benefit of having an offset is you can grill depending how you have it built either over the firebox of some are able to put coal in the chamber.
I chose to buy a vertical smoker instead of replacing my offset and I must say I would do it again in a heart beat. In my opinion LSG or Humphrey's are two of the best vertical smokers out there, Humphrey's units for the most part are reverse flow units and they are incredible. LSG makes both offsets and vertical chambers and have many many years of experience in the business. A vertical smoker, I find the efficiency is incredible, you can set and leave if you want to cook over night, cross contaminating is very easy to handle and take care of I personally don't find it to be an issue.