As with many things in sound the answer to which is better is ... it depends on a few things which is the best to go for and which sounds best.
First thing to consider is fire rates and frame rates and if you audio engine is frame rate dependent.
Granular systems (e.g. 100ms sounds 6-7 variants played consecutively) this works well if the rpm of the gun is well below the frame rate or your audio engine is capable of playing sounds between frames (frame rate independent). If you try to play a high rpm weapon with a frame rate dependent audio engine it plays back un evenly.
For example if the frame rate is 30 Hz and the rpm of the gun is 25 the audio engine will try to play a sound every 1.2 frames. This is can't do so what you'll get is something like 4 frames where a sound will play then it will skip a frame and then start playing again for 4 frames etc.
In my humble opinion this can make the gun sound weaker and not as convincing for the player. Also if the game is not at a stable frame rate this can seriously effect the sound.
Games like Battlefield use the frame rate independent system build into layers. Max Payne 1 and 2 however had a frame rate dependent rapid firing guns, and if I remember correctly so does Death Rally for iOS.
Looping is a good solution if you have a frame dependent audio engine and the fire rate of the weapons is high. if you know the clip length of each weapon then you can make one 'taka taka taka' sound that covers the whole clip length, this way you can bake in the 'swell' of the gun sound into the wav file which adds nicely to the sound.
'Looping' systems are usually broken into 3 parts. A single shot when the player pulls the trigger, followed by the loop as the trigger is help, then a single shot tail when the trigger is released. The good things about the looping system is that it is easy to integrate and the quality is guaranteed each time. The bad thing is that the loop is the same each time which can get annoying and if you want to change the fire rate of the weapon for any reason then you have to remake the sound. This is the system we used on Alan Wake American Nightmare in general we got pretty good response for the gun sounds.