I still say it is great to even be having this type of discussion, but I grew up in the '80s when nothing was fast (until the late 80s with the revival of the 5.0 Mustang and Buick GN).
Thing is... is anyone making a hot-rod truck right now? No. We have the (c)Raptor and Dodge and GM have... nothing.
Anyway, one reason there are more L's running faster is b/c of the sheer numbers of them. If there are 28k L's and 9k 10s, then even if the 10 is a little quicker (talking 1/4 mile) stock-for-stock, there are more L's out there in existence to modify.
I personally think the L is just set up better from the factory as a 1/4 mile racer...assuming that is your cup of tea and you are going to mod whatever you buy. Lighter vehicle, auto trans is easier to drive and not as hard on driveline parts, and it is so much more cost-effective to mod, given the factory supercharged package.
The 10 is a top-speed vehicle, has better brakes for sure, and is nicer inside (and I like the L interior/seats). From the factory, the suspension may be better... I don't know. My 2 L's never kept the stock suspension for long, and my 10 doesn't have a stock suspension either.
I've had 2 L's (still have one) and its a great truck. I've had 2 10's (still have one), and its also a great truck.
If I was building one as a 1/4 mile racer, it would be the L, for the reasons stated above. With a pulley, a tune, a larger (more efficient) heat exchanger for the intercooler, a cat-back exhaust and Nitto DR tires (not a real drag tire at all), my '04 made 391rwhp and ran 12.81 @ 109mph. That's ~$1200 in aftermarket parts, and didn't compromise anything as far as driveability. Toss on an aftermarket blower (half the cost of a set of heads for the '10), it made 460rwhp with a very conservative pump-gas tune... basic dragstrip math says it runs low-12s in that trim on radials, without going into the motor. Toss on slicks and a converter and its in the 11s easily.
But I don't get to the track much anymore, so I drive the '10.
We'll see what the '10 makes power-wise after these Gen IV heads/intake get installed.
But given the '10 weighs a lot more (which negates the stock-for-stock advantage in power for the most part), there are less 10s around, and the 10 is far more $$ to modify reliably... is it really a shocker there are more modded L's running faster than 10s?
They are both awesome vehicles, and hopefully the Big 3 will start building a new generation of hot-rod trucks again.