On June 17th, 2013. Intel officially announced the Intel 520 solid state drive had reached end of life. The last day for retailers to order the 520 was October 15th, 2013. There may be retailers that have unsold inventory. The 520 was replaced by the 530.
The Samsung 840 EVO was released in July 2013. It has become the number one best selling ssd in the world. Part of the success was due to Samsung securing lucrative contracts to supply Dell, Lenovo, Sony, Apple, and other computer companies with OEM versions their ssd's. The 840 EVO performs exceptionally well and has a proven track record. Last month Samsung released their new 850 Pro model. The 850 EVO model will be released in the very near future.
I normally recommend Samsung ssd's and mention that SanDisk, Crucial, and Intel are also worth considering. Intel usually commands a premium price.
For all practical purposes consumers and casual gamers will not see or feel any performance difference among modern 3rd generation SATA 3 6Gb/s solid state drives. There would have to be a very heavy workload for a user to notice a difference.
I maintain the ssd database listed in a sticky at the very top of this forum section. Here is the link:
http://www.johnnylucky.org/data-storage/ssd-database.html
Scroll down to the brands and models you are interested and follow the links to the technical reviews.