Ready to order a Spikes complete upper, but am undecided between melonite finished inside and out or chrome lined with phosphate finish. Was ready to buy chrome, but read that melonite offers better corrosion resistance than chrome. Any opinions?

I would think that unless you plan on using the gun in very extreme conditions and rarely clean it, the chrome /phosphate finish should be gtg.  The military still requests chrome lined barrel with phosphate outer, and they use/abuse their guns alot harder than anyone else. But in the end it's your gun so get it the way you want it.

If you want the information you're looking for I'd get it straight from the horses mouth.  Talk to Spike's about it, and I'm sure they can answer all your questions concerning melonite vs. chrome.  I think either would serve you well.

I bought an M4 upper with piston kit, so my route was a stainless barrel with the Melonite finish.

Chrome lined barrels are nice. That was the option I chose with my DPMS upper that I had on order, but I'm not storming the beaches at Normandy with it... thinking it through now, it was a reason why I waited a year, and probably going to wait another one (I also opted for the Cryo barrel, which I think wouldn’t see a lot of benefit with the lining). Besides, you will lose some accuracy with the lining. Most people do not need it, nor will they ever.

Now, for the accuracy differences between the two, I have no idea. I've heard that Melonite is also superior, but did not look into it further.

I talked to spikes befor I ordered there top of the line piston upper. I then called my brother that has a masters in engineering and his BA is in materials. After getting info from both places I went with the melonite. Get  the best one time and no regrets. Spikes promised me great accuracy and a damn long barrel life.

Melonite is superior to chrome lining in every way.  It is the same metal treatment Glock does to their barrels and slides.

What is the cost of melonite vs chrome?