Going dry only makes sense if your water temperatures are very cold, and also when you are willing to dive. It will also depend on your tollerance to cold water. I live in the North East so I do not know the water temperature in Colorado, but 50 degress is tollerable in a good fitting semi-dry, and if you can tollerate the cold.
I own both, so I will talk about both and you can decided which is better. I picked up a virtually brand new Whites fusion drysuit with a pinnacle evolution undergarmet, dry glove system, drysuit hose, and a carrying bag for $500 (one fourth of retail value) at my dive shops annual used equipment sale. For that price it was a given that you would buy the drysuit because it was cheaper than most semi-dry suits. However, a drysuit is not easy to dive in, be perpared to not like your first dives ecspecially when your feet are above your head. It takes practice to dive horizontally in a dry suit and sometimes requires ancle weights. I would suggest renting before buying to see if you will like it, but do not stop after one, two maybe three dives. My experience just shows that checking out used sales may help you find a great drysuit for a low price.
Now, My Dad and I are dive buddies, and he just bought the Cressi Lontra. We were looking into the semi-dry concept as a way to dive in warmth during the summer. As of now the Cressi is selling in the mid $200 range, this makes it close to the price of a Scubapro form2 wetsuit (not semi-dry). This again comes down to water temp, but the Lontra is not the best semi-dry suit out there. Many say that the Scubapro Novascotia 6.5mm suit is better. Even though it is not as thick as the 7mm lontra many people say that is seals are almost as good as those found on a drysuit. Because of the seals divers report coming out of the water with only the lower half of thier legs wet. This them seems like the suit to go with but it is around $550, more than twice the price of the Lontra.
If you are going to buy the Lontra there are a coubple of things to know. Firstly, the seams are not fluid sealed which is a way of significantly reducing the water which comes in through the stitching. Also, for the best seal Cressi offers a hood which is seperate from the suit (even though its in the same picture as the suit). Another thing to note is that the best semi-dry suit depends on how well it fits you, the cressi might not be tight enoguht to act as a good semi-dry and vise vera for any other suit.
So thats my rather long two-cents, I hope it helps.