50 Megabit (Mbit) is 6.25 Megabytes (MB) per second. 250 Mbit is 31.25 MB per second.
It depends on what you need it for. If you do not do heavy uploading/downloading or a lot of streaming video, or you do not run servers of any kind (web, email, ftp, podcast, other), I'd do the 50 Megabit one.
Just make sure you have unlimited data for downloading. Some ISPs may cap you at 10 gigabytes a month, or something like that.
If you go either route, make sure you can swap to the other (without too much hassle) if you need to. In other words, I would buy the faster cable modem (what the 250 deal requires) and use it for the 50 connection. If you want ot upgrade, you won't need to buy a second modem.
I would buy not rent modems. You end up paying for multiple modems with renting, and never own one. Ask what brands work best with their service (and both throughput speeds). See if Amazon carries the modems for cheap, or see if your local computer store carries them cheap. If you are American, try Best Buy; and definitely get the replacement/Geeksquad warranty (for 3 years?). This saved me LOTS of money in the past.