I am looking at purchasing either a Mid-length or a M4 rifle parts kit from Del-Ton (both flat top) for my first build.
Does anyone have experience with either or both of these? What advantage do these configuration have?

Thanks
Lance

Mid-lengths have a longer gas system that enables them to be more reliable that the carbine length system.

Quoted: Mid-lengths have a longer gas system that enables them to be more reliable that the carbine length system.

Smoother perceived recoil- if there is such a thing with .223,  I have never had an unreliable carbine so I can't see that as much of an issue-reliability that is.

both are good setups

mid vs CAR=...try a search

It's commonly misunderstood what is meant by reliable with the middy carbine disscusion. Middys cycle slower hence longevity of parts. You also don't need to worry so much about having the proper buffer,extractor spring and insert etc. If your carbine is setup properly it will be just as reliable just on paper your parts won't last as long. YMMV

The midi can take a bayonet.

I've had a Delton mid length for about 2 years.  It works great and was easy to put together.  Remember the middy is a heavy barrel where the M4 should be the lighter government profile barrel.  I have a qual rail, VFG and flashlight on my Stag M4 style carbine and it feels the same as my heavy barrel midlength.   I don't really notice the difference in recoil in the two rifles because I run a carbine buffer in the midlenght and a H2 buffer in the Stag.  The midlength is suppose to be easier on parts, but I only shoot about a 1000 rounds a year so I guessing that is not enough to matter.  David

What length of the upper receiver is actually determined as mid length?

Quoted: What length of the upper receiver is actually determined as mid length?

The gas system (tube) determines length.

There is a discussion still on the front page dealing with this:

www.ar15.com/forums/topic.html?b=3&f=4&t=380257

both are good setups
you will be happy with either one