Credit unions are not-for-profit and owned by account holders so they are not out to make money. Not every credit union is great but there are many that serve customers better than megabanks.
Credit unions that participate in Shared Branching make it possible to make deposits/withdrawals at over a 4,000 different credit union branches. My main CU is 8 miles from home but there's a different one I pass on my daily commute - and I use that one more than my own via Shared Branching. Best of both worlds - I get all the services I need from my main CU and have my accounts there, but I have the convenience of using a different one right near home.
Many CUs are part of national ATM networks as well.
So the criteria I would use is
Go to
http://www.asmarterchoice.org/ to search for CUs near you.
Verify eligibility. Many CUs are opening their membership to people who live, work, or worship in a particular geographical area and some like Alliant take anyone no matter what. Traditionally CUs had pretty limited eligibility but that is changing.
Make sure you find one that participates in Shared Branching if you want to access your accounts in person in different cities.
If you're a big ATM user, find one that participates in a national ATM network so you can avoid fees.
Check the hours they are open to see if it's a good fit for you.
Of course, you'll make sure they have an account type and fees that are a good fit also.
All that being said, I would not worry too much about choosing one based on the fact that you might move somewhere else someday. I wouldn't sacrifice other things just for that reason.