I'd say both are solid games... it really comes down to are you more a fan of modern warfare or World War 2 stuff. Alot of people are trying COD5 right now but the COD4 servers are still going strong.

Personally I found it hard to find a weapon with really good balance in COD 5. All the weapons are very situation specific... and encounters in multi player lean heavy on certain weapons trumping other weapons. Which increases strategy while taking away a little from skill.

Graphics in COD5 are kick ass and single player wasnt bad but just felt like an update of 1-3.

Again though that is just my opinion.