If the condition of the A-arm are the same then IMO it is 6 of one half a dozen of the other.
If one set needs work (i.e. bushings) the other doesn't, then I would lean toward using the best condition set.
Using the complete 911 front end is simple and straight forward, just bolts on. All stock sway bars are fixed (not adjustable). If you want an adjustable sway bar you need to go with the thru body type bar and weld tabs to the 911 arms (after you cut off the original sway bar brackets. Street car won't need an adjustable sway bar thou. Yes 911 and 914 A-arms use different T-bars that are not interchangeable. There are more choices for T-bars for 911 A-arms than 914 A-arms but there are still choices. 21, 22 & 23 mm bars are still available new and you can find 20mm used.
911 front end has bigger T-bars than stock 914 and a bigger sway bar so it will be stiffer than the 914 set up would be. This might be a good thing if you plan on stiffening the back end to match the front. If you were not planning on stiffening the rear end you may want to go with the 914 sway bar and A-arms. You have to decide how much ride quality you are willing to give up to get better handling.
Personally I am a fan of the thru body style sway bars and used 911 struts with 914 A-arms but I was not going for just a street car. I also went with a 19mm adjustable sway bar and 22mm front T-bars, 160lbs rear springs, plastic bushings frt & rear, and Koni Sport gas shocks all around. This is not the most comfortable riding 914 out there, well at least it wasn't when it still ran
Jim