I'm not sure what an Antec 900 is like, although I do own a P180 and can give my view on it. It's my main system case, and has a few ups and downs.
Pros:
1. Excellent cooling from CPU perspective, 2x 120mm exhausts. Works extremely well with my cooler (Arctic Cooling Freezer 7 Pro) due to the size of the heatsink and the fan blowing straight through it and straight out the top of the case.
2. Very quiet system due to sound padding, I'm running 3x 120 mm fans (low setting) and when I enable SMART fan for the CPU I can barely hear anything.
3. Removable HDD bays, you can remove the center one and mount a 120mm intake fan at the front of the case, or you can attach it to the HDD bay itself to make a VGA cooler (however bad point is that you would probably have to remove the HDD bay anyway with a large gfx card because it won't fit in with the hard drives in the middle section - meaning you're restricted to one HDD bay at the lower area, which holds 3-4 hard drives and can become VERY hot if you remove the bottom fan next to the CPU for better cable management).
Cons:
1. PSU at the bottom sounds a novel idea, but it's not. The amount of airflow restriction because I have to stretch a motheboard cable and a molex cable over the middle of a case to reach the top of it to plug in the DVD-ROM and motherboard plug is unreal. If you can get a PSU with VERY long cables, then you might be alright as you can run them up the back of the case and stick them to the back panel, but if not, cable management in this case will be horrific.
2. The 120mm fans are quite thick fans, it can cause issues in numerous circumstances. I've found that I basically had to remove the bottom 120mm fan next to the PSU because the cables kept touching the fan it's that cramped in there. The one at the back of the system could prove problems with certain CPU coolers because it pushes out on the motherboard quite far, so you might have to get some thinner 120mm fans (there's about 1.5cm between the back fan and my freezer 7 pro). Another point about the fans is one of my 120mm fans actually started making VERY loud noises after about 2 weeks, although just probably a dodgy fan when I got it.
3. Airflow in general is quite horrific, and the case gets very hot sometimes due to restrictions inside the case, if you don't run a lot of stuff inside the system like SLi cards and lots of hard drives / dvd+cd drives then you might get away with better temperatures, but as soon as you start adding more hardware there's just no where to put the cables properly and airflow suffers massively.
4. Another PSU issue is that not all PSU's will fit in the case, there's a mounting bracket over the PSU and some just will not fit with it properly. I have an Antec Smart Power 500w PSU and even that was an extremely close fit, I had to bend the bracket a little to hold it in place properly (and the cables are very short, this from an Antec PSU I would have though would work better in an Antec case).
5. Very heavy case, you could probably save yourself a few quid instead of going to the gym and just bench press your P180 instead it's that heavy.
Also...
As im having 4 hard drives installed in this system i wanted a case that has 2 x removable caddies with each holding 3 drives so i can fit 2 in each and have a air space between.
The middle HDD dock only holds 2 hard drives in total, but does have spaces in between so the air flow is fine. But I'm letting you know in case you actually wanted 3 hard drives in the middle hdd bay.
Overall it's an "okay" case, but if I had a choice again to buy it or not, I wouldn't, at least not for the price it's still going for, there's better cases for it's price for sure.
