LED's have a limited life-span and you shouldn't read the 50,000 hours etc. blurb when installing in a vehicle.
Inconsistent voltages (engine cranking with lights on for example) kill LED's much sooner than you can imagine.
Having said that, I'm still a big fan of LED's but the point is - don't buy a set of 2, by spares as well.
Second point worth mentioning is brightness and that's measure in Lumens.
Different LED chips have different values of Lumens.
I always prefer bright LED's and of course, the brighter they are the more watts they use.
CREE based LED's are one of the brightest and there are more COB based LED's on the market now as well.
CREE tend to be more projector based LED's.
COB based LED's are LED's much closer together on the PCB and appear more like one large LED!
I'm sure this example offers bulbs far too long but it's just for example and you can find shorter bulbs.
I was hoping to pick-up a pair of used headlights, crack them open after heating them briefly in the oven and install some LED flexi-tubes. I know there is perhaps a bit of space around the main beam assembly worth considering but I haven't had a chance to look close up yet.
As an alternative and I haven't measure sizes yet, these caught my eye and reminded me of a previous project I did on my Volvo headlights.
I will post some results when I've completed what I'm doing.