I'd go with the battery tester. But to be honest its the person using the items who makes the real difference. No good having all the tools if you don't know what to do with them or how to diagnose. I'm good friends with GEE TEE and his cocker knowledge is supreme and the team always ask the "orracle" (pun intended) for help, but its his diag skills that are the most valued.
Absolutely
Primary problem with electronic guns, and so many new batteries aren't up to it. If you can then test batteries in the shops (many 9 volts are boxed with the contacts poking out) and it lets you confirm if there is any life before touching the gun