I personally think the Rob Allen Tuna is one of the best entry level guns and has a lifetime warranty. Ideally you should try a few different gun setups from fellow spearos so you can get a feel and some feedback on different models, this will get you some direction as to what features you want in a gun. In example, you might prefer loading a pneumatic over a euro band gun with steel wishbones when you slice your fingers open, or a single band vs dual or triple band guns with only one wrap of line for faster reloading. I prefer open muzzles over closed muzzles because it gives me the option to use a larger diameter shaft and not have the crimp sleeve hang up in the muzzle that was designed for a smaller shaft, but if freeshafting then a closed muzzle is probably a better option unless a bungee or rubber band is rigged to keep the shaft up against the open muzzle.
In my geographic location we are diving in warm/tropical water so euro guns without a loading butt (chest load) are uncomfortable to load since we rarely wear neoprene or chest pads in the water, so your water temperatures may have some determination on what style gun is more comfortable to load for you. Gun length is an enviromental consideration, longer guns >110cm are more suitable for blue water environments with good visibility where effective range is crucial whereas shorter guns <90cm are more preferable when diving shallow water with less visibility or shooting fish underneath rocks and ledges.
Hope this points you in the right direction, dive safe.