The 100-PX will certainly give you more flow than the 50-PX, I am just saying that I don't know that it is worth the additional cost.
Estimating how much flow you will actually get is tedious and not really worth the effort, but if you are interested here goes:
The pressure or head loss of a section of piping is a function of the ID and the flow rate through it. Attached is a chart for schedule 40 PVC piping.
https://www.engineeringtoolbox.com/pvc-pipes-friction-loss-d_802.html
Your tubing ID is not the same as 3/4" SCH 40 PVC, but as an example for 3/4" piping at 420 gph, the head loss is 10.5 ft for every 100 ft of length. The trick is estimating the equivalent length. It is not just the 38 ft of straight run. Fittings, bends etc and sudden contractions and expansions will add to it significantly. It is a tedious calculation to capture all the losses. There are some on-line calculators but they often lack all of the components. In my industry, I often just double the straight run as a starting point. It should be conservative in your case using hose. Using this approximation, you have 2 x 38 or 76 feet of equivalent length. That would mean you would have 10.5 ft times 76/100 or about 7.6 ft of head loss. Any elevation change would be added to that. Lets say it is 4 ft. Then your total head requirement at 420 gph would be 11.6 ft
If you wanted 420 gph, you then would want to look at a pump that can deliver 420 gph at 11.6 ft. The amount of flow a pump can deliver is a function of the system head. As the head or back-pressure increases the flow will decrease. A lot of aquarium pump manufacturers only list the max flow (at zero head) and the max head (at zero flow). The better pump manufacturers will provide a curve that shows the delivered head as a function of the flow rate. If there is no curve provided you can estimate it assuming a linear (straight line) head curve between those two points.
Pan World provides curves on their web site. Use the 60 hz curves for North America
http://www.panworldpump.com/products_03.php?cs=3&ss=15
Looking at the head curve for the 50 PX, it will only deliver about 7 ft at 420 gph (26 liters per min), so it should deliver a little less than 420 gph. The 100 PX would deliver 420 gph at about 13 ft of head, so it will flow more than 420 gph.
You have to do a trial and error solution to match your system head losses with the pump. If the flow is increased to 600 gph the head loss through the piping is now 20 ft per 100 ft, so for 76 feet you would have about 15 feet plus the 4 ft of elevation gain or 19 ft. At 600 gph, the 100 PX would only be able to deliver about 9 ft of head, so you would not get anywhere near 600 gph. Probably something like 500 gph if you ran all the calculations, so you might gain 100 gph by going with the 100 PX over the 50 PX
The point of all this is that the system head loss really snowballs to very high levels as you push the flow rate. When this happens, you really need larger pipe. That is why I am saying that for a 3/4" pipe system, the 50 PX is probably the best choice, but there is no reason you could not use the 100 PX if you wanted.
The PX-X series pump curves are on the site as well. The analysis method is the same. They have higher flow capabilities but only at low head. At high head requirements, the PX pumps will actually produce more flow.
