Difference between SanDisk Extreme SDSSDX-120G-G25 & SanDisk Ultra SDSSDH-120G-G25

Q1:
Why there's difference in terms of performance between:
a) SanDisk Extreme SSD 120 GB SATA 6.0 Gb-s2.5-Inch Solid State Drive SDSSDX-120G-G25
 and
b) SanDisk Ultra 120 GB Solid State Drive SDSSDH-120G-G25

Q2:
for (a), MTBF is 2.5million hrs.  Anyone can provide the
MTBF for (b)?

For (a), amazon url give 510/550Mbps write/read while
for (b), amazon url give 270/280Mbps write/read.

Both are SSDs, why such a big gap in performance?


For a, I refer to the url
http://www.amazon.com/SanDisk-Extreme-Gb-s2-5-Inch-Solid-SDSSDX-120G-G25/dp/B006EKJCWM/ref=sr_1_1?ie=UTF8&qid=1356796395&sr=8-1&keywords=SDSSDX-120G-G25

For b, I refer to the url
http://www.amazon.com/SanDisk-Ultra-Solid-State-SDSSDH-120G-G25/dp/B005F30IJK/ref=sr_1_3?ie=UTF8&qid=1356796395&sr=8-3&keywords=SDSSDX-120G-G25

Q3:
Can both the models be inserted into Lenovo X220 laptop for
Win 7 boot-up ?