Hmm...it depends entirely on the kits you're comparing. 8GB/stick non-ECC produces more errors than 4G/stick RAM. 'IF' you'll need 32GB then 8GB/stick is obvious. Next, RAM's 'performance' is determined by the IC's used (Rated frequency, CAS Timings, voltages, etc).
Also, if 'later' then I'd probably look for a very popular
kit that's going to have a long
Example of a good 2x4GB that 'should' be around for a while -> Corsair CMZ8GX3M2A1600C9 - http://www.newegg.com/Product/Product.aspx?Item=N82E16820233144
; note there are several color versions which all have the same IC's. The CMZ8GX3M2A1600C9 kit has proven itself to be stable and have a low error rate.
Keep in mind all
RAM throws errors (1-2 errors per month) and 8GB is higher. While 'ECC' RAM can correct these errors non-ECC cannot. This brings me to the next topic of 'use,' if you're a professional then I'd consider a Xeon and ECC RAM the cost isn't too much more but the end 'product' will be error free.