Wanting to keep my flip up sights on so what would be a good optic to use and still be able to use the flip ups with if wanted/needed?

Kind of vague on what you'll be using it for.Quality mounts will be just as important if you will be removing and replacing and want to maintain Zero.Think Larue.

If you actually intend on using the backup sights with whatever optic you choose then you will need to be using a red dot or acog. If using a scope you will likely have to remove the scope if you want to use the flip ups.

Flip up front and rear? If just rear Is flip up, the eotech is easier because the center dot will sit above the front sight post without a riser. The aimpoint pro comes with a mount that will cowitness pretty close to the same as the eotech. But the other aimpoints usually need a mount bought separately. This will mean u need to choose between absolute cowitness or lower third cowitness. Absolute cowitness means the center dot will line up with the iron sights and lower third means the iron sights line up below the center dot. But if u look through the iron sights the red dot will line up with the sights. If u use a scope or magnified optic, such as an acog, then the iron sights will not be visible through the optic. At very low magnification the front sight may appear as a slight blur. Therefore if u intend to use a magnified optic, u will want to use a quick release mount to still be able to utilize your iron sights if needed or ur optic fails. I haven't had much experience with any other red dot sights but I hope this helps u understand it a bit more. And sorry about the u instead of you I'm on my phone and it's a habit and didn't notice until I wrote all of this.

Flip up front and rear

Have several different scopes I could use on it just wanting to do something different and maybe a eotech or something

Eotech and aimpoint are the most common other than scopes. Burris also has an AR sight. I believe they offer it in 1x, 3x and 5x now. I prefer aimpoints but a lot of people like the eotech reticle. It just depends on what fits you the best and how much you want to spend really.

I know of no scope with magnification that you can co-witness with AR-Iron Sights.

An Aimpoint Red Dot, Eotech or Trijicon ACOG are what comes to mind. There are separate magnifiers available to assist.

Well, it's not so much the optic as the mounting system. If you're wanting to keep the flip ups on there as a backup, you're going to want a QD mount for your optic. Throw a couple levers, take it off, and you're good to use the iron sights.

Place optic back on when you want to, put the irons back down, and you're ready to rock. Now, taking it on and off again will affect your zero, but some mounts are so well made (ADM, Bobro, LaRue) that your shift in impact will be minimal.

If you don't want to do this, you've always got the option of having some sort of flip-to-side mount for an EOTech with magnifier.

I've been thinking about picking up two of these to mount a scope, then you could flip it to the side when you don't want to use the scope but it would keep your scope in line for other shots. I'll have to see if it holds zero or not though.

http://www.combatsportsupply.com/ampfliptosidemount.aspx

Also if you PM whatsup he said he worked for a scope dealer or something along that line, he might have some good advice for you.

I would not trust ZERO on a flip to side mounted scope, but that's me.

Usually the flip to the side mounts are only for a magnifier.

Personal preferance comes into play between Eotech and Aimpoint.Some prefer the reticle of the Eotech for fast target acquisition.I prefer the Aimpoint for less cluter in feild of view.Both optics would cowittness and serve you well.

Put a Vortex SPARC on mine and love it like a fat kid loves cake. Has a 3X magnification attachment (screw on) if wanted. perfect co-witness with iron sights (fixed front and flip up magpul rear) After quite a bit of time, 8 months or so, the battery crapped out the last time at the range - no problem iron sight dead on at 100yds, and tagged perfect M.O.D. 'minute of deer' groups, lol