P300 case has a little RGB lighting and can plug in to your motherboard so you can set the LED RGB colour through your motherboard. Although I couldn't find any information from Phantek on how to connect the aRGB to your motherboard, so it's a bit sketchy and I couldn't verify the proper setup for the RGB lighting.
Anyway, I have made some comparison. You can try to take a look:
The G21 is slightly bigger than P300, meaning you will have more room to work with (the CPU cooler height clearance is 160mm for both G21 and P300, however, so some tall coolers might not fit in both case).
G21 has GPU clearance of up to 410mm, (P300 only has clearance of 330mm) so you have less restriction on the GPU you can put in your case (You can easily put a 3 fans GPU card in G21).
As for cable management, P300 has a compartment to manage all the cables from PSU, whereas G21 lacks that. P300 can hide most of the cable in their dedicated compartment, and allows you for a cleaner build.
In liquid cooling, TT G21 can support 360mm radiator at the front and 120mm radiator at the back, whereas P300 can only support up to 280mm radiator at its front and 120mm radiator at the back.
Case fan cooling, well, I guess you had know by now, but just to point out. G21 can install up to 6 120mm fans, 3 at front, 1 at top, 1 at rear and 1 at bottom. P300 can fit 4 120mm fans, 2 at front, 1 at rear and 1 at top. As for 140mm fans, both can fit 3 of this fan, 2 fans at front and 1 fan at top.
RGB lighting, P300 has one, but G21 doesn't.
With the above difference, I hope this will made clear on what you are aiming for. You can check their specification at below links. TT G21 actually has a more comprehensive manual, so you can check it out.
