What do you want to use it for outside of SU? If gaming, consider a PC. PCs are much easier to upgrade and add or remove components from with competitive pricing. If you want a good work computer, then get a Mac. They have good stability and usability, but you're generally going to be locked in to the hardware and Mac software (even though you can run Windows on a Mac, but for that matter you can create a mac clone and run OSX on a PC too).
Whatever works for you. I'm not a fanboy of either of them, they both have their place. My house has i-devices and PCs both. I just prefer the PC for the aforementioned reasons; I can multi-boot linux flavors and windows versions and replace any component as necessary for an upgrade within the limitations of the motherboard. You can make it as powerful or as efficient as you like. With a self-built PC, the upgrade costs decrease after the first build. The LCD monitor, CD/DVD, HDD drives and case can almost always be cannibalized for the next system. Often the sound card (if not built-in) and power supply can be as well (if a high quality one was purchased initially). That leaves only the CPU, motherboard, GPU and memory for each upgrade. A considerable savings. My last build for internal components was less than $1000 for an i7, 24 GB mem, 1TB HDD and a GTX570. The rest of the parts were salvage.
If I had to buy a pre-built system, I have also heard that ASUS is OK, and my folks have an HP that works very well. You'll likely need to go through and uninstall any pre-installed trial or other software by the manufacturer, there's usually plenty of it and it has nothing to do with making your computer work.
JMO.