First, I would just play through an episode one of each to make sure there are no obvious encoding flaws.
Since you already have the files, it makes the process much simpler. I'd say to just go with the screenshots and decide which one looks better to you. Sometimes the finer differences can be a difference in preference, so if one isn't clearly better than the other, then just pick whichever one you like best.
Bitrate is usually a good estimate of the quality level, but it is just as likely that something with a (slightly) lower bitrate but a lot of work done on it will look better than a higher bitrate release. Also, check the codecs. If they are the same then there are no issues. If they are not the same, than the one that is bigger in size isn't necessarily the one that is better quality. Newer codecs usually maintain better quality even at lower filesizes.