The best answer should come from someone having experience with both. At a glance AMP seems better as you'll get better rates and a wide choice of platforms.
I'm a happy Ninjatrader/Ninjatrader Brokerage customer, though. On average very stable including high volatility periods. Execution and fills are great as far as I can tell, although I'm a piker trading small size of course. Never had any issues with disconnecting. Customer service is on average pretty good I'd say. Had some correspondence with them as late as this week and answers are usually pretty fast.
I was a bit fed up earlier with their antiquated reporting format (PDFs), but now they've launched their electronic portal offering analytics similar to Tradovate (which they bought), so that's a big improvement. I've also been a bit fed up by their software at times, basically considering it a software written for traders by programmers and being very slow to implement very basic stuff. As a result, I use a bit of custom indicators to fulfill my needs and requirements.
But I'm mostly settled now and am overall happy, so I'm not shopping around for anything else at the moment. I do have a lifetime license so I get fairly good rates as well.
If you want to use NT I think you'll be pretty happy with NT Brokerage, but keep in mind you need to lease/buy the software if you want to trade through it. At least if you want full functionality.
So, bottom end, AMP may be the cheaper and most versatile option.