I would like to hear the opinions on which one would you get. If you could have ONE, which one, and a short idea of why. I realize it is not exactly apples to apples but let go with with the choices. I realize rifles are a personal matter and what is good for one person may not be the same for another.  I have no experience with sig rifles and would like to learn the good and bad before I buy one.  

I am concerned about reliability and replacement parts/accessories.

I appreciate any opinions.

Thank you for taking the time.  

JC

I can't speak to the XI as I have an MCX.
I picked it up because I have plenty of AR's and was looking for something different but can still take some common parts. Going through the 40+ page thread most of the opinions on it are very positive.
I've had a fairly good experience with mine. The only issue I had with mine was a failure to load. But I attributed it to the older again mags I was using. It's run just fine with the PMAGS I have.
I personally like the skeleton folding stock it comes with. I wrapped the top of it in para cord and out here in the desert, it doesn't get hot sitting in the sun. I also swapped out the trigger for a geissele as the stock one, while serviceable was quite heavy.

Overall I'm very happy with it.

My LGS has a few old-stock versions of the discontinued P556 Classic  pistols (with an AR tube and the Sig brace from the factory)  NIB. I noticed the P556 Classics only because I was considering an MCX as an SBR project and really liked the Swiss styling and lower price of the discounted model P556 (no Swat models), but wasn't too crazy about having to stick with an AR stock after my SBR paperwork is approved. I also really like the MCX's folding skeletonized stock.

If the P556 Classic (10" barrel/Swiss handguard) were thrown in the mix, would it move to the top of the list...

P556 Classic versus XI versus MCX?