If you can only afford to get one of them right now I would recommend getting the IR temp gun first.
You can probably find both of them for $60-70 combined.
I'd say a temp gun is really handy, and much more convenient as well. And most of the time you would not need a scale unless you are trying to keep track of their growth or meal sizes for documentation purposes for future transactions. But if you do have a ball that is off feed, I would really suggest getting a scale to monitor his health and weight.
You need a thermometer you can trust. IR guns are no more or less trustworthy than any other. They have more issues in use as well. Scales are a good indicator of health weigh gain or loss especially with older snakes. If you have a reliable thermometer an ir gun is just a connivence get the scale if you don't ir gun that is a reliable one.