First, I don't believe the MCP50X will work with the Glacier R220 unless you are using the pump free standing. The MCP50X is not compatible with standard DDC tops.
Second, you have a couple of standard misconceptions going on in your thought process - namely that "bigger must be better" and that "push/pull must be better than push only". If you do a quick review here https://www.xtremerigs.net/2015/02/1...-round-2015/5/
, you will find this is not always the case. The GTS and GTX were made for different applications and usage, and both excel at their intended usage. The GTS was designed for quiet rigs and will outperform a GTX (which is designed for high fan speed usage) up to ~1350 rpm. It will have a nominal difference in performance to ~1750 rpm. Until you get into fan speeds that will make your ears bleed, the GTX doesn't separate itself from the GTS. So, unless you are want is to build something that sounds like a vacuum cleaner, the GTS is going to be the better performer.
You will also notice that P/P has essentially no performance impact on the GTS. So, you aren't going to gain anything by going P/P, and it will be louder at the same performance levels. However, with a 420mm on a CPU, you will likely never have to break 1000 rpm, so it won't be much of a concern.
Personally, I think the GTS is a far
better choice for your application - better performance, less noise, and retains the ability to use the Halos.