Which one do you prefer and why?

I am going with Metal Slug because I like to jump on tanks and run into guys and not die right away.  Also, Metal Slug Advance was so bad ass.  I love Metal Slug's graphics style, weapons, vehicles, and characters. 

Contra is classic and I love it a lot especially Contra 3 (SNES) and Contra (NES).

Also, if anyone wants to discuss these two series then lets get to it!  Both these series are classic and deserve as much love as they can get!  Anybody have any news on any new iterations for these two series?

metal slug, i could never get into contra.

Metal Slug.

Metal Slug is way more fun. Gets my vote easily.

Contra was a very fun, old-school game that turned into a mediocre series as it was milked for all it was worth. Metal Slug has kept the same style of gameplay throughout all of its iterations.

My verdict: Metal Slug

Up Up Down Down Left Right Left Right B A Start, that's my choice =/

Contra, no question.

Metal Slug focuses too much on saving near-naked bearded men! ;)

Ahhh that is my favorite part of the game besides jumping into a Metal Slug/SV-001.  If you don't rescue those bearded men then you don't get the bad ass shotgun, machinegun, laser, etc..

But...but I can has double Spread Shots in Contra III and Contra IV right at the beginning of both games. Can't beat that!

Ah yea hard to beat unless on some Metal Slug levels you get to start with the Metal Slug! Tough call!