It sounds like they both do the same thing, which is make it easier to hide during combat. I do NOT like the NPC's Terminator radar ability at all. I think it's ridiculous how easy and accurate they can pin point your location. Which one should I use, or should I use them both? I know in the SM combat read me, it talks about being compatible with Attack and Hide and recommends to adjust some values in the ini file. Is it like gonna make it twice as easy to hide during combat if I use both of them?

There is some difference between the mods. I have not read the readme's lately to see what feature is affected by which mod, but this is what SM Combat hide's ini has to say about it:

"SetGS iAICombatMinDetection -47 ; Affects ability to lose track of target by sneaking in combat. Default value is -47,
; Vanilla default is -50, Attack and Hide is usually between -45 and -40."

Attack and Hide only has the one game setting change. It doesn't have any other features, unlike SM Combat Hide which is a lot more sophisticated.

If you are ever curious about what is under the hood in any given mod, or comparing two mods to see what the differences are, I highly recommend downloading this tool:
http://www.tesnexus.com/downloads/file.php?id=11536

Using it was what led me to creating SDR. I was comparing all the stealth mods to try and see if I could merge them all together to reduce my .esp count, and discovered none of them behaved the way I thought (or wanted).