The Corsair TX 850w is manufactured by CWT which has a reputation for good quality PSUs. Corsair also have some PSU models manufactured by Seasonic who has a reputation for building excellent quality PSUs.
The Silent Pro 800w is built by Enhance Electronics which is a OEM like CWT manufacturing PSUs for many other companies, but does not sell any PSUs under it own name in the retail market. I would say it's reputation is equal to that of CWT.
Both are good PSUs, but based on reviews that I've read the Corsair beats it by a small margin.
i wouldnt base this choice solely on reviews. Reviews dont show what the PSU will be like in a few years time. Coolermaster don't have the best reputation for PSU's. Corsair on the other hand don't seem to ever put out a bad model. I would stick with the corsair. 850w is not necessary, but if you can afford it, you may as well get the 850w. Better to have more now than find out you need more in the future.