I love my MiniST 2, received back in March, and have seen several comments on various forums about non-ST owners wishing they'd bought the step-thru. IMHO there is no downside to doing so and it comes in very handy if you have any joint issues or if you add a rack and big basket on the back, or just carry large things now and then, needing to swing a leg over with the non-ST version. There are no structural issues; I'm not tiny and it's as solid as a rock.
Your choice of course, and the new price is pretty great - the only thing I'd do is have it checked over thoroughly by a bike mechanic if you're not one, because Rad is now forced into using substituted parts on some models due to supply shortages. It'll still be fully warranted of course, but you want to be sure everything is aligned correctly and up to snuff. At a minimum, check to make sure all fasteners are torqued properly.
And since you're new to Rads, FYI you should know that all current Rad models (except the new Rover 6, I presume) have a problem with the key that locks the battery in place - all of them can be unlocked and stolen with a $7 uncut blank key easily found on eBay. Hard to believe, I know, but I carry a blank on my keyring to show to other Rad owners I come across. That's the reason for the little U-lock on my Mini at the top of the battery. Worth knowing if you end up parking it in crimey places sometimes; you can either add theft protection / alarm or just take the battery with you if it's concerning. My little town is not a problem, but the engineer OCD in me insisted on addressing the problem.
Aside from a minor squeak on the front end (which Rad helped me fix over the phone when I couldn't find the source on my own), I've had zero problems. In fact, I just checked tire pressure yesterday and after two weeks it hasn't even dropped a single PSI (but I did Slime both ends shortly after I got it).
If you buy a Rad model of any kind, be sure to hit up Ryan's discount page here on the forum to get an additional $50 referral discount (if it's still applicable) and get him a little bonus from Rad at no cost to you.
Nice little bikes, for sure. Here's pic of mine with accessories added (except for the new GoPro addition):