My experience of one-sided pedals is that they’re not very good flat pedals, as well as being more difficult to get in/out of when using the cleats. They’re also quite expensive - you can get excellent flats/SPD pedals at the cheap end of the spectrum. I started with one-sided SPDs when I took up MTB as did a few of my friends and we all ditched (some for flats, some for SPD) them as soon as we could afford new pedals.
Big, grippy flat pedals with stealth rubber shoes are awesome and help you to learn good technique to stay connected to the bike. If you learn how to use flats properly they’re just as secure as SPDs in my experience - I find them a much more fun experience. 5.10s are the go-to flat pedal shoe but it’s worth trying a few on to get a decent fit.
If you find yourself racing or doing long, less technical stuff then the (small) extra efficiency you get from SPDs can be nice. Fit is more important here as the shoe is supporting more of your foot so try on as many pairs as you can!