Windows 7 professional or Windows 10?

You know only you can answer this question. What I recommend you do is research WIn 10 and compare its features to Win 7 and see if you think you'd like it. There are literally 100s of blogs and forums on the internet about Win 10 that will give you information to assess. Some people are going to say "you should" and some are going to say "you should not" and it will be a subjective opinion of what they like, not having anyway of knowing what you would like.

Remember you have one full year to take advantage of the free upgrade. That's plenty of time to research before jumping in the fire.

The key thing you need to research before you make the jump, is make sure your hardware devices are compatible with Win 10. Audio cards, video cards, wireless mouse, wireless keyboards, printers, scanners, cameras, and any other hardware. Thus far, the only issues I've seen have been driver updates needed for hardware devices so you can save yourself some headaches by making sure your hardware is comptable

You could have both, by creating a separate partition on your main hard drive and installing win 10 on that, afterwards everytime you turn the PC on it will ask you which one you want to boot. And if you choose to remove one. Go to the OS you decide to keep and delete the other partition.