You are smart to consider gas logs before building the fireplace. Its much easier and neater to run the gas line now, than later. Even if you don't use gas, at least run the runs.
I installed ventless logs last fall and really like them. No, we don't need to keep a window open. No, we don't have problems with high humidity in the house. And yes, its on quite a bit in the late afternoon and evenings.
I don't know how they would work in a masonary box. Check with a reputable dealer of vented logs for that one.
There are dangers with the amount of heat rising up the front of the fireplace. This affects placement of a mantle. It can be altered somewhat with a shield. There are minimum clearances for each brand and size. If you know the log set you want, get the manual and make sure the clearances are followed. Even if you don't know the specific brand, at least get one for the size of logs you are contemplating.