It is been my experience that a square or high enclosure makes it difficult to create a proper temperature gradient. You just can't have one end around 88 degrees under a basking lamp and/or heat pad while the other end is close to 70 and that's what garters need. IMO a long enclosure with more floor space, and no higher than 20 inches or so, is better. That tall square enclosure is better suited for arboreal species. I just don't think it's the best dimensions for garter snakes.

I would use a 20 gallon long before I would use a tall 50 gallon any day.