I have been researching this somewhat lately as i myself plan to get a similar upgrade sometime in the near future.. What I can say is that the Renntech Pulley is a modded stock pulley while the evosport is a completely new pulley which looks clearly aftermarket... although this by itself does not say much.

The Evosport dampened pulley shows a 30 WHP gain on their dyno for their slk32 and c32 and will most likely provide an equal result on our srt.. this is more than stated by renntech at an almost equal price for the pulley set. As for the ECU the renntech shows a 25 FWHP gain which is likely more than the one shown by evosport. Both of their pulley + ecu set come out with a difference of only about $200...

Now it boils down to which way you want to go... To me and most other users Renntech is a largely more reputable company that has been in the business for a long time... they get their products from a division called MKB and are barely different from those offered by Brabus or kleeman as all of them get their products from MKB. Evosport designs their own products, but the ecu tune will be done by Powerchip in this case. So as the price is so barely different it all comes down to which way you want to go and... what you prioritize... to me it seams as Renntech is slightly more reliable while evosport has a slight edge in performance...

As one other user stated that he would go to the tuner closest to him... I suggest the same thing... as this decision is difficult to make. Just make sure that if you do decide to go with evosport you fork out that extra $500 and purchase the dampened pulley as it will make it easier to sleep at night.

One more thing... I have no proof of some of the things i stated it is largely a combination of speculation and logic.