whats better the regular n42 head on my L28 or a E31 head?...and would the E31 bolt on with no modifications?

Neither quick, nor easy, nor is there any single concrete answer. The volume of cylinder head discussions on this and other sites would fill several fat books. You'll need to do some searching here and decide which will work better for you.

+1!

check out with hybridz.org also, it might help more.

Depending on what you're trying to achieve, the N42 head used with flat top pistons makes for decent performance on pump gas. The E31 , with the same flat top pistons, will give you a higher compression ratio. It will most likely require larger valves with some valve unshrouding to get proper breathing with the L28. You may need octane booster with the E31 to prevent detonation, depending on the octane level of the fuel in your area. You can also play with head gasket thickness to raise and lower your compression ratio. Just my two cents.

n42 = bigger valves, more flow with no modification

e31 = slightly higher compression ratio (i think on my f54 with flat top pistons it's like a .3 to 1 difference)

i chose the n42 because it's less hassle