That depends on what you intend on doing with the lab map. If you want the lab map to get a specific color, saving up for a paint brush for a dream pet is better. If you want a lab rat to end up as a BD pet, and don't care about gender/color/species, and are willing to keep zapping, regardless, the lab would be your best bet. It just depends on your resources, and you're intended purpose.
Well I painted a pet before I got the lab map, ended up getting it eventually for quite cheap and I've been saving up for paint brushes ever since. But if you get a good "zap" and it's not a dreamy you can always trade it for a pet you actually want, so I'd do lab map first the paint brush. But with Key Quest now lab map prices are going down fast.
I think if you're aiming for a specific pet of yours to be painted a specific color you should buy the PB. Your Uni may turned into a Skeith or something you hate. I have four lab maps and over time it's definitely worth it. Especially for those lab only zaps (MSPs, Peas, Jellies, Chocos) Ultimately it's up to you though. GL!
Yea i also think that the lab ray is a good idea, just keep in mind that it has the chance to decrease your stats as well... so if you are looking for a BD pet, get the PB because your pet's stats could go down a bit...