I think the Arc Midi is very versatile in comparison to the 300. I like the removable steel HD cages and there is many more options for cooling I've installed both air and water cool systems in the Arc. I recently outfitted the Arc Midi case with dual loop... Drilling 4 holes in the bezel, for mounting a 1540mm size radiator was only modification needed. I removed the front grill in this photo, so you can see how clean my execution was.
http://www.mnpctech.3dpixelnet.com/picture_library/fractal_design_arc_midi_dual_loop_ces_2012_mnpctech.jpg
Asus Sabertooth P67 Rev 3
eVGA GeForce 560Ti
Patriot 8GB Division 2, VIPER EXTREME, DDR3
Patriot SSD WILDFIRE 120GB
SeaSonic X750 Gold
EK-Supreme HFHigh Flow CPU Water Block - Acetal+EN (Nickel)
EK-FC560 GTX V2 - Acetal + EN (Nickel)
Koolance RP-402X2 Dual 5.25" Reservoir
Koolance Bleed Pipe Kit for RP-402X2
Black Ice® GT Stealth 240 Highest Performance Radiator
EK-CoolStream Radiator XTC (140)
Enzotech (BRHF-G1/4-12-45) 45 Degree Rotary Fitting G 1/4 Thread
Enzotech (BRHF-G1/4-12-30) 30 Degree Rotary Fitting G 1/4 Thread - Matte Black
PrimoFlex Pro LRT White Tubing -1/2in. ID X 3/4in. OD
Danger Den UV White Delrin 1/2" (3/4" OD) Tubing Clamps
Feser One F1 Cooling Fluid - Black/UV Blue - 1 Liter