Regarding Resident Evil 5....http://www.tomshardware.com/reviews/resident-evil-5,2409-4.html
Sorry, but the response from the team is not very exciting: simple answer for both, [no] new content, or shaders, [or] effects added to Direct[X] 10.
Benefits for the DirectX 10 users: nothing special, no enhancement.
In other words, Resident Evil 5 is really a DX9 title because no DX10 effect are used. Think of Halo 2. Halo 2 is a DX9 game, but in order to make gamers switch over to Windows Vista, Microsoft designed the PC version of the game to require DX10 in order to run.
I will assume AMD updated thier drivers since the review to reduce the amount of performance hit the Radeons take when playing under DX10.
No similar statements from the developer of DMC 4, but apparently you are not the only person who does not see a difference between DX9 and DX10.http://www.gamespot.com/pc/action/devilmaycry4/show_msgs.php?topic_id=m-1-45051911&pid=938687
I'd say it is similar to RE5; a so called "DX10" game, that is in reality just a DX9 title with DX10 coding, but no new effects.
If I were you, I would play the games under both DX9 and DX10 and select the one that gives you the best performance, because visually there ain't no difference.