Value is best defined by the price per gigabyte in a storage device. This used to be held by smaller capacity drives but now higher ones are starting to offer roughly the same cost per gigabyte. With a price of $215 and a capacity of 512GB, the Crucial MX100 ends up with just $.42 per gigabyte to offer a solid value. In addition to offering a nice capacity for the price, the drive also offers some solid performance thanks to its custom Marvell controller with roughly 550MB/s read speeds and 500MB/s write speeds. If you don't want to spend that much but still get a slightly higher price per gigabyte, there is the Crucial MX100 256GB model priced around $115
Performance in solid state drives is reliant upon the memory chips, the drive controller and what interface version it uses. The problem is that most of the drives are nearing their maximum potential from the SATA III interface. In fact, most people would probably not see much of a difference between most of the drives but if you want the fastest performance available, the Samsung 850 Pro pushes the absolute limits of what can be had in the interface. The drive features read speeds of up to 550MB/s and write speeds of almost 520MB/s. The downside is that it does cost a bit more with prices for the 256GB version running around $200 but it does feature a 10 year warranty. The next closest drive in terms of performance is the Intel 730 Series but it features just 240GB of storage space, a five year warranty and it is a bit more affordable at $190.
If you want the most capacity possible out of a drive and are not too concerned about costs, then the Samsung 840 EVO 1TB drive is an obvious choice. This drive uses a new type of triple cell NAND to offer higher density memory chips. Performance is still quite good thanks to the use of the new Turbo Write capabilities from the Samsung memory controller. Speed can reach up to 540MB/s for reads and 520MB/s for write operations. Of course if you want the absolute best performance, than the Samsung 850 Pro 1TB drive does get a bit of an edge and longer warranty but the price jumps up to near $700.
A number of PCs that use Intel processors and chipsets can accelerate a hard drive with a solid state drive through a caching system called Smart Response Technology. Essentially, a small SSD drive up to 64GB in size can be used as cache for frequently used data to improve performance. In this size range, the SanDisk Ultra Plus 64GB drive offers a good level of performance especially with data reads that are critical for the caching system as writes are typically still restricted to the speeds of the hard drive it is caching. The only downside is pricing is still around $70 which isn't great at a per GB rate. In fact, a good alternative is to buy something like the Crucial MX100 128GB drive for just $5 more and have both a 64GB cache plus an extra 64GB of fast storage space.
If you don't want to spend a huge sum to get an M.2 SSD drive for your laptop or desktop system, then the Crucial M550 256GB drive offers a nice compromise between capacity and cost. With a price tag of around $170, it offers a price per gigabyte of about $.66/GB. This makes it an affordable option but it should be noted that the performance is still limited to the SATA bus speeds unlike the Samsung XP941. This means it offers read speeds of up to 550MBps and write speeds of up to 500MB/s which is still good but not that much better than a traditional 2.5-inch SSD drive.
The new M.2 interface brings with it the ability to have huge performance gains over the previous mSATA cards. The reason is that the new M.2 can use PCI-Express lanes instead of the SATA interface for greater bandwidth. If you happen to have a motherboard or laptop that uses PCI-Express lanes with the M.2 interface, than the best performance drive available right now is the Samsung XP941 which features a native PCI-Express interface for the SSD chips. Samsung touts read speeds of 1170MB/s and write speeds of 950MB/s for the 512MB model which is nearly double what the Samsung 840 EVO 2.5-inch drives can achieve. Sadly, using the drive is quite costly as it is priced around $500. In addition, many systems still can't boot directly the operating system from this drive just yet.
There are a large number of desktop motherboards that are now featuring the M.2 interface on them. The thing is, the software is not well designed to use the PCI-Express and NVMe to get the full performance advantages. Having said that, the M.2 interface is very convenient for adding a small drive for use with Intel's Smart Response Technology. Transcend's MTS M.2 64GB drive is an ideal choice for this thanks to is relatively affordable price of around $60 and its small size that means it can fit on almost any board.