Why did I chose this particular one? It was the best "bang for the buck".
Unlike tripods where the differences are easily measured (min height, weight, size etc.), tripod heads are not.
With the exception of the weight they are capable of handling, the differences are much more subbtle making it much more complicated to properly select one.
Things like how smooth they operate, how precise the adjustments are, the material they are made of, are all extemely important but much less tangible.
What feels "ergonomic" to me, may not to you.
How did I arrive at this particular model
I looked a three manufacturers that had the reputation for first class equipment:
Bogen-Manfrotto
Acratech
Really Right Stuff
Off course there are many more. For example Gitzo has superb equipment but I felt they were out of my monetary league. Another manufacturer that makes first class equipment is 360Precision however they specialize in panorama heads and I was looking for a general use head.
From Bogen-Manfrotto I looked at the 400 series heads. They have several models of the type that I was looking for. The differences were the size (height), weight rating, construction material and camera adapter style. Prices ranged from less than US$50 to more than US$300

From Acratech, I really liked the Acratech V2 Ballhead model. It is built as solid as they come, capable of handling lots of weight and the are very precise and accurate. They have 3 models ranging in price from US$250 to almost US$400.

Really Right Stuff sells very high quality equipmentn to professional photographers. They have primarely three heads: BH55 BH40 and BH25. All equaly superb in construction, the only differences being the size and the weight rating. The prices range from US$145 for the BH25 up to US$575 for the BH55 with the camera plate.

After reading both print and online reviews, asking many questions, and trying different ones, it became obvious to me that there were two main factors that dictated the price of a tripod head: weight rating and material.
Off course quality of construction, adjustment controls, leveling aids, quick connect systems, all influence the ultimate price. But those two seemed to be the primary ones.
After comparing them, I felt the Really Right Stuff heads where head and shoulders above the others. But, would I be willing to pay US$390 for a mid size BH40 head? Not really. Sure I could have selected the much cheaper BH25 but it felt to me, small for anything much bigger than a "point and shoot" camera.
That left Acratech and Bogen-Manfrotto.
The basic Acratech Ultimate Ballhead had all the features that I wanted: solid, precise, easily adjusted with full motion of the ball. It even comes in left handed or right handed models. It is also, and this probably sounds silly, the best looking of the bunch: I looks like a modern sculpture... The price was around $US275 including the camera plate. Expensive but, because I liked it so much, I couldn't dismiss yet...
Bogen-Manfrotto, makes great tripods and there was never any doubt which manufacturer I was going to buy my tripod. Their heads, however, are not as well known in the community of photographers. Sure, everyone knows they made heads, but pros typically stayed with other more established, specialty makers.
Bogen-Manfrotto has more than a dozen of heads. All kinds of prices, all kinds of sizes. From minuscule little jewels to massive behemoths capable of handling almost 30lbs of weight.
I tried several with different cameras. I experimented with different tripods. Every time I compared the different models, I found myself leaning towards the 486 or the beefier 488 model. It was rugged, smooth, not to big and very well made. It was not made of magnesium like their top of the line models which meant it would weight a little more. It had a weight rating of 16lbs (12 for the 486) and came with 2 spirit levels. The camera plate was included and had a very helpful circular scale in degrees that would come handy when shooting panos. There was however a big difference in price, The 488 was twice as much as the 486 due to the weight rating. This primarily due to the higher weight rating.
After seriously contemplating both my talent and my bank account, I decided on the 488. Price, including an extra camera plate, US$165.
The truth is, the Acratech was the one that I REALLY wanted except that. in the end, I couldn't sell its virtues to my personal Chief Financial Officer: my wife.