If you prefer Android over Apple iOS, then yes, it would be a good move. What is it that the iPhone can do that the Android cannot do? I can't really think of anything except a better texting app amongst Apple users. However, I'm sure there are many things Android can do that Apple cannot do. I would think the Pixel's camera has a slight edge. Battery life? Probably better with the Pixel 6a, unless you have the Pro Max model.
If you make the switch one thing you might notice is the Apple has a smoother scrolling experience. The Pixel 6a still uses a 60Hz scroll rate. Not that it's bad but you might notice it's not as polished as the Apple. But I'm sure after a couple of days of not going back-and-forth among devices you won't notice it.
As to the price, that's pretty good, especially if you can keep your iPhone (though why would you?). Does it require trading in your iPhone? Because I would think you could get at least $300 for you iPhone if you were to sell it on one of the online sites like Swappa. The 6a was (is?) on sale for $399 recently which means you could get the 6a for $99 (net) if you sold your iPhone.
As to going with one of the bigger brother Pixel 6's you will gain a couple of features (and some size) but I wouldn't think that would be worth $200 to $600 more you would pay to get one.