IMO, go direct vent. The top vent (regular chimney) is just that, a regular chimney pipe - a single pipe carrying combustion products out of the home. When using a regular chimney, the combustion air comes from somewhere else, usually leaky windows, doors, bathroom fans etc. To me that means you lose comfort (it's drafty) in those areas where you're pulling combustion air into the home. And the pipe gets hot so you have to respect the proper firecode installation rules. On the other hand, direct vent chimneys are double-wall pipe. The center pipe carries the hot combustion products out of the home and the surrounding outer pipe brings in the outside air. That cuts down on the drafts at doors and windows, plus the outside of the vent pipe runs cooler, so it should be safer from a fire standpoint (you still install it per the firecode). The safety of direct vent IMO would be higher. I don't think there would be any major difference between installation, cost, appearance of the fireplace, or flexibility between the two options.

I have a Fireplace Xtrordinair natural gas insert (made to go in an existing fireplace), but FPX also makes fireplace units for new construction. No problems w/ the FPX in the two years I've used it. I've posted several times about this unit on this forum in case you want to do a search. FPX offers fireplace units in various sizes & BTU outputs. You buy the firebox size you want & choose a faceplate (several styles & finishes available). I haven't bought a fireplace, so have no knowledge of how you "finish" one, but I believe FPX has an alliance with a company that makes mantles, etc. if that's the look you want. My info is two years old, but FPX did offer an optional remote control to turn the unit on & off, operate the blower speed (if it has one) & control the flame height. Other options included a thermostat to turn the unit on automatically (like a furnace) & a ducting system to share the heated air with an adjacent room. The FPX can operate as a radiant heater in the event of a power outage, but would need a backup power source if you wanted to run the blower. In terms of maintenance, FPX recommends an annual cleaning & inspection. I confess I waited until this year to have that done, and the service guy only needed to vacuum out dust & clean the inside of the glass -- said the burner, etc. looked fine. Other manufacturers probably have similar features. The various manufacturers offer different interior & exterior appearances, so, if possible, visit dealers in your area to see the ("live") units in person so you get a feel for what you like. Check the BTU ranges & efficiency ratings. Your choice may depend on if you want the unit to be a heat source or more for ambiance. Some units come with a blower; on others, it's optional. Most units will work with LP or natural gas, but ratings might vary w/ the fuel source. If you want to operate the unit in the event of a power outage, make sure it's capable of doing that. One trend in fireplace units is to have a huge, "to the floor" viewing area w/ the controls "hidden" on a wall elsewhere rather than built into the unit. "Town & Country" is one brand that does this. I know nothing about the company or how convenient it is to install/operate the unit, but I saw one on display at our local stove store & the appearance was impressive. In addition to FPX -- which is part of Travis Industries, along with the Avalon & Lopi brands -- the Mendota and Harman brands get good reviews on this board. Another option might be a freestanding gas stove rather than a fireplace, but, again, what "look" you want -- as well as your budget -- may determine your choice. The only other point I can think of right now is, work with a good, reputable local dealer. There are lots of options out there. Most of the manufacturers have websites, with varying degrees of usefulness. I found that with FPX, for example, the brochure I picked up from the dealer was much more informative than the website (again, that was two years ago). The FPX site does have a great "design (photo) gallery" in case you want to get an idea of how their unit styles fit into varius room designs (traditional, period, contemporary, informal, etc.). Good luck w/ your research.

If i have a 40” linear gas fireplace with a wood mantel that is 24” above the top of fireplace can i hang my TV on the wall?

I've had both vented and ventless from Petersen Real Fyre and they both looked very good. Be careful about measurements and make sure you get the largest that will fit, so they fill the fireplace. Is your fireplace designed for direct vent, with a separate intake for combustion air? If you don't install glass doors it will be easier to make them look realistic, but will also be much less efficient. The gas logs installer is supposed to block the flue open (for safety), so you are not supposed to be able to open and close it after they are installed. Bruce