My opinion:

BUT - both are comparable in performance... not much difference...

No real reason to have a 3TB hd internal to either... I would drop that down to 1 TB and invest in externals.

RAM - get the IMac with 8 GB and upgrade to 32 from OWC - about half the cost of Apple Ram.

I would much rather be running on a version of UNIX which is way more bulletproof than anything running on a PC these days, especially if it is from Microsoft.

When that PC is creaking and tired in a year, the IMac will be moving along the same as the day you brought it home. It will run for years with zero issues - very few Windows users can say that.

If you are getting a desktop (more bang for the buck), you will either save a lot of $$ or get a more powerful system if you consider a Windows system. For your budget of $2500 you can get fast i7 quad core cpu 32 gb ram, dual 30 bit 24" displays, workstation graphics card that will show 30 bit color when editing in Photoshop (if you go that route), and more internal storage than any Apple product can offer. With any motherboard that supports Intel Rapid Storage technology, you can use a small inexpensive SSD drive to cache a 1 TB system drive, (90% of the performance of an SSD only solution, but only 25% of the price), and 3TB storage drive (possibly larger in this budget), and you can use high quality high duty cycle enterprise drives as opposed to the junk you by from Best Buy. Something to consider. BTW, I have been specifying and building machines since 1983. I also have supported Apple products.

Mike, I have used the CT3442 with their CB50D and found it more than adequate, plus it allows the legs to be folded 180 degrees so you don't have remove it and install it each time you use it.

I would have no hesitation to take it or the Sirui hiking or backpacking.

I have the big brother - the CT3472 which have used with a D300 (cropped sensor), a 600mm F4 and a 1.4X extender - total focal effective focal length of to 1260mm get this image at 1/10 sec - the only blur you see is the bird's leg moving.