Killzone 2 has the most immersive gameplay of the bunch currently, people complained about the controls until they found out there are more than 1 button layout and was "fixed" which I figured was to stop all the whining. You shouldn't invest in something like the COD series until MW 2 since it is just a generic shooter. Killzone 2 is not a run n' gun game either so you will probably enjoy the strategic refreshment it serves. I base this on the fact that you own MGS4. The online of Killzone 2 is very fun and versatile. You will find no "twinks" (people who seem to have a clear and large advantage over the average player) like you will in COD4 (ie: the abuse of stopping power and alike).