i think this will depend on how much noise you can handle, and what you expect to do with it.
if you are looking for the best cooling, all all cost, i'd go for the slk-900, with a tornado . i have the slk-800, with a 92mm tornado, and i love it.
dollar for dollar, the sk-7 is great "bang for the buck", and gives the most for your money, but if it's not enough, you might have to spend a lot more for the slk-900, to get minimal increase in performance.
i also have used the ax-7, which is a nice heatsink, but doesn't do quite as well at lower cfm...... the all copper sk-7, and slk series will do good with low cfm, and great with high cfm.
i have reached 2640mhz on air with the current slk-800/tornado combo i use now, but this tornado is a bit loud. i also have some loud 120mm exhaust blowholes, and 3 more tornado 80mm intakes, and 2 52cfm intakes, so, sound is not going to bother me much. the noise does get annoying at times, as my pc is in the living room, and my wife and kids don't like this noise when they are watching tv....
it is tough for us to pick your choice for you, but we must consider your priorities, as well as cost. usually small gains in cooling cost a bundle more than the "bang for the buck" options....
if you want the best cooling, and money isn't a big object, go for the gusto, but if you are into "bang for the buck", the slk-900 is just too pricey. either way, i will get one sooner or later...thermalright is coming out with a heatpipe version of their copper heatsinks, and this will most likely drive down the slk-900 prices, and at that time, i will be looking for the slk-900 a version...which is the newer slk-900 with the socket "a" clip, instead of the "bolt on" mounting system. i do , however, have concerns about the weight of that heatsink , hanging from the ziff socket, so, i may just go for the bolt on in the long run..
i hope this has helped you decide. sometimes we buy bang for the buck, and later wish we had taken the plunge and bought the best.......other times we kick ourselves for paying too much for such small gains...
i seriously doubt you will be disappointed in either purchase, but i do suggest the tornado if noise is not a concern.