What he said
Is it worth you more to sell the old one then to keep it?
If you've spent much on the old one you are unlikely to recoup the difference than for it as standard
Is it your first gun? You may regret selling it just because it was the first gun and as memories, you may not be sentimental or the memories may not be so fantastic
I see by stalking your post history that you have bought an etek
You ought to be able to learn how to look after the etek and resolve most problems. A backup as an alternate to easily switch to can be a disadvantage preventing you from looking after the primary and maintaining it
I still own most of the guns I have ever had, and sometimes take a number of them out. The benefit is I can play with whatever takes my fancy, and I often find myself prefering one for months and then getting one of the old ones out just for a change.
The advantage/disadvantage is how easy it is to switch when there is a minor problem, I have found myself starting a day with problems on a particular gun, I don't want to be held up with everything else to do at the start of the day - checkin, airing up, filling pods, chronoing, meeting people. Then I can start the day with what works first time, then get back to the original. Usually any issue can be dealt with in 10-30 minutes, but it's a matter of taking time out and diagnosing why the problem is there. That makes the spare an advantage and disadvantage - it can get you into the game, but it can stop you learning and sorting problems first time, or stop you doing the maintenance that you should have done at home