WELCOME!!!
Great start.
Fuzzies are perfect. Here's a link to help guide you along the way with prey sizing: https://ball-pythons.net/forums/show...ing-Guidelines.
The tubs I use for hatchlings on up to ~600 grams are 18" x 7.5" x 3.5". You're definitely in the ballpark with sizing. You can't really go wrong with either choice.
In terms of temperature gradient when you get the heat mat/thermostat set up, the two you have are basically the same length. You're not really changing anything up for the little guy when it comes to that aspect - you're basically just giving him an extra ~3" of width.