The X200s uses a 2.5" inch hard drive - so without an adapter a 1.8" SSD will not fit.
There is really no difference between a 2.5" SSD and a 1.8" in terms of lifespan. Here's where the design matters: Is the SSD MLC or SLC. SLC will have longer life and better performance* but it costs more.
I would buy a 2.5" SLC SSD (if you can spend the extra money - do) with upgradable firmware. How do you know you can flash the firmware? Check the manufacturer's website. If they offer firmware downloads then you're good.
*Are all MLC drives bad? No. The Intel X25-M uses MLC and it's a very good drive - but Intel also did some great design work and because of that and because it's from Intel it is expensive. I'd rather get a SLC SSD.
If you would like more information about SSD architecture and the differences between SLC and MLC take a look at this Anandtech article
. It is well written and informative.