Thanks for reaching out and welcome to the community!
We'll gladly share some insights here with you, and we're also going to move your post over to the Camping board in order to tap into the collective wisdom of the rest of the community as well.
In terms of staying warm at night, a sleeping bag is important, but so is a sleeping pad, a layering system, and you (specifically your metabolism and how you create heat at night). A sleeping bag, in and of itself, does not generate warmth, it only insulates what is inside it. As such, if you are cold and you get in a cold sleeping bag, it will not get warm until your body heats it up. And if you are a person who struggles to warm your body up, that can be a long process at best, and impossible at worst.
If someone is struggling to stay warm in a 30° sleeping bag on a 60° night, I first look at a the sleeping pad they are using and then what they're doing to get warm and stay warm in their sleeping bag. If you have a sleeping pad that insulates against the ground, then I recommend thinking about your pre-bed routine. Adding in some jumping jacks, a brisk walk to use the restroom, or even sit-ups inside your sleeping bag will get your heart rate up and your blood pumping. That can be all it takes to warm up your sleeping bag quickly and keep you warm all night.
Layers are also an important piece, however, you want to make sure your layers aren't so tight they impact your circulation, and thus your body's ability to warm itself. Personally I have to be careful with my socks. If the cuff around my calf is too snug, having 'warm' socks on can actually make my feet colder!
Lastly, it may just be that you need a warmer sleeping bag. In that case, I've always gone by the rule that it's easier to dump heat than create it. My go-to sleeping bag is a 17° bag that I use for extended three seasons. I tend to run warm anyway, so often times I just have the sleeping bag wide open and draped over me like a quilt. I also often open the zipper up from the bottom as if my feet are hot I'm likely not going to get much sleep.
All of that is to say: all other things being equal and space not being a consideration, I'd vote for the 15° bag!
Hopefully this helps, thanks!