I'm trying to decide what to replace the stock M4 handguards on a Colt carbine with, M500A or a M962XM07 (with rail mount tape switch).  This carbine would also be converted to a LW gas piston, and I already know I can mount the M500A over the piston cup (with minor modification).

Stock Colt M4 Handguards are 6.7oz, plus 2.5oz barrel nut = 9.2oz total.

Can go with a GP ARM-R-C rail, which is 10.7oz, and add a Surefire M962XM07, which at 12.3oz would be a net gain of 13.8oz

Or, I could go with the M500A, which at 20.8oz would give a net gain of 14.1oz

Weight difference between the two options isn't very much.  I don't really care for rails or vertical
grips.  I would lose the front side sling mount going with the M500A, but I plan on a single point sling anyway.  Rails would also incur the added weight of ladders/panels.

The only other question would be price, $280(rail) + cost of the M962 vs cost of the M500A.

Which option would you take?

I went with SF M961C which is similar to the M962XM07 except mine is without rail mount tape switch and I only got the thumbscrew mount.

I already have a LaRue 9.0 FF rails on my Colt 6721 (custom middy) tactical carbine so I picked the M96XX model type because it could be mounted in different right, left or lower rails.

The only change I would make on mine is to upgrade to a LaRue light mount.

It's up to you, if you want the feature which could change the light's mounting location the M962XM07 (with rails) is a good choice, if you like the fixed position the M500A is fine.

Everyone will have different opinion on cost, some care about it, some don't.

In my case if I have money at the time, I go for it, if not I just choose the lesser cost.

Going with the rail system will allow thumb activation if you go to the combo tail unit which many prefer.
It will allow greater cooling.
It will free float the barrel.
It will allow easier access to the piston if you have any issues with it.
It will weigh less.

The M500 will give you Nav LEDs and has no exposed wires. If the NAV LEDs are like my M900 they will activate if wet and stay illuminated for hours.

Id pick the ARM-R plus rail mounted light.  In fact that is what I am doing on my LW build. I decided that I did not need the throw of the 9V Surefire M900 but wanted more than a P61 could deliver.  I opted for a Gladius to reduce weight further.

The rail would offer easy piston access and free float, plus I wouldn't need to mod the M500A either.  I would go with a rail mount tape switch, no good way to activate to a rear push button w/o a vertical grip.

like the others above I would/did go with a rail mounted weapon light

I would go with a rail light.  There are so many other mounting options if you don't want the rail, look at GG&G fsp mount and others.  I used a 500 for a while but found two problems, it wouldn't fit in my patrol rack and I hated the swithces being so far foreward.