It's most definitely not the ONLY difference.
Splash:
+extra 120mm rad support
+easier to route the tubing
+room for a dedicated pump + res inside the case
+drainport / fillport option
-less options for HDD's (only 2 x 2.5")
-no ODD
-not so well finished, very "industrial" look
-heavy
M1:
+multiple HDD options
+beautiful finish, light as a feather
+enough room for decent air cooling
+ODD possibility
-very little room for a dedicated pump and reservoir (basically only external reservoirs possible)
-no drainport / fillport
-gets really crowded with tubing for CPU and GPU
-only a single 240mm rad will fit
AIO coolers will fit both easily of course.
But as for me, the extra 120mm rad in the Splash was a dealbreaker.
Only difference in water cooling the case i meant.
The Splash will have lower temps just becasue it has the extra room inside for more air flow, I have a full loop in my build and I'm using a independent pump, M1 res, dual 120mm rad and a full motherboard waterblock and to tell you the truth my temps are always under 55c on the GPU and 60c on the CPU and thats on max load and this is with a 4.4ghz OC on the CPU to and its summer here and i dont have AC, Don't forget the splash uses a itx power supply as well so really you have no head room for a huge OC any ways.