I have both. My Surefire P6 is very old, and I have a couple of drop-ins for it. It has type II anodizing, which is to say it looks pretty worn. I don't recall precisely how old it is, but it predates the Web...
For drop-ins I have a Ultrafire or some such 4 mode drop-in, and one of the new Bugout Q5 preminum drop-ins. Except that one is back at the shop since it didn't fit my old light quite right.
Brightness wise they are pretty similar (the Ultrafire is not as bright, but the Bugoutgear Q5 is similar to the P3Dq5. Patterns I can't compare yet since the Bugoutgear didn't work much when I had it (due to the mechanical issues). The Fenix pattern has a nice gradient. It has good throw, but not a sharply defined hotspot, so it is useful for close in use as well. I find the pattern more useful than the tighter more defined and throwy hotspot of the Olight T15q5.
The user interface is very different. The Surefire is a momentary non-clicky, twist to lock on. This is good for some things, but not good for a multimode drop-in. The Ultrafire is multimode and it has memory, so each time it is turned on it advances to the next mode. Not great.
The Fenix has a much more sophisticated user interface. It has two "cycles". One cycle is turbo / strobe. The other cycle is low / medium / high / SOS. It uses a reverse clicky and has no memory (beyond a couple of seconds), so each time you turn it on it comes on in the first mode of the cycle. The cycle is selected by tightening or loosening the head slightly. Tight is turbo / strobe. Loose is low / medium / high / SOS. So you can select turbo or low before turning on the light by adjusting the head. To advance modes within the cycle you bump the clicky when it is on. To turn off or on push the clicky till it clicks. Harder to explain than to do, but it allows you to select things in a fairly efficient way.
The Fenix is type III anodize, very durable and hard to scratch. I'm not sure what the new Surefire P6 has for finish. Mine is very soft and worn.
The Fenix comes with a holster and spare parts. My Surefire did not. I'm not sure what they come with these days. A premium drop-in for the Surefire can cost about as much as the entire Fenix P3dq5 does.
The Surefire will give you an incandescent drop-in option, as it is generally not hard to switch drop-ins.
The Fenix is a lot smaller, and is cylindrical. The Surefire has a larger head. The two lights are quite different in their feel. I like both of them, but the Fenix is easier to every day carry.
I'm not familiar with the L1 but aftermarket drop-ins are usually far brighter than Surefire LEDs, and the L1 is a one cell light, so it is not quite comparable to the P6 or P3Dq5.
-- Alan