Hey! I have a bit of a long answer to this. I'll start with the short one: Garden Select vs Essential. I prefer Essential. Or, I should say, my rats do. Oxbow knows what they're doing with feeds - but there is little benefit to one over the other, it's a personal preference and that of your rats'. Garden Select is vegan, non-GMO, and a larger variety of source ingredients (greater range in vitamins and minerals). However, since Garden Select is vegan-friendly, the base proteins differ (fish vs soy). Some rats (like mine) will turn their noses up at it. I have 8 rats currently. One (1) of those rats will eat Garden. The others won't touch it.
So... onto the blocks (aka "lab blocks"). Firstly, they are referred to as lab blocks because they're used to maintain the health & weight of - drumroll - lab rats. As much as that's a whole other can of worms, it means the food takes the animals' long-term quality of life and nutritional needs very seriously. Oxbow being one of the best (there are 2 or 3 other brands high on that list as well). That said, it is recommended to supplement pellet diets with a variety of other foods, both for the health benefits & enrichment of your colony. There are lots of great lists of rat-safe supplemental foods, some of which can be given daily and others are only recommended a few times a week.
I have nothing against people who want to manage their rats' feed more strictly than blocks allow. However, it's not viable for everyone and there are trusted options available to you (like Oxbow). If you want to be mixing your rats feed yourself, you have to put in a lot of time (mainly in securing enough accurate knowledge), effort, and money to balance their diet with longevity and quality of life in mind. Lab blocks, however, will ensure your colony is getting the correct nutrients with some added buffs from supplemental foods or vitamins.
At the end of the day... depends how keen & confident you are on learning and keeping it up. There is nothing wrong with a lab block base as long as you are not solely relying on it, and are offering your rats a variety of tasty & healthy snacks throughout the week.