One of each...
I've discovered over time that you really need one of each if you want to cover all the bases...
The UV dye method is excellent where there are multiple possible sources of refrigerant... i.e. near the compressor... its hard to tell exactly which fitting or seal is leaking with a sniffer. Don't misunderstand, it can be done, but its a challenge. The dye will appear (glow) right at the source of the leak. The drawback of dye based systems is the initial insertion of the dye into the system. If you don't have the proper tools/manifold, you'll have to find a can of refrigerant that already contains the dye.
The sniffer is best when looking for an evaporator leak. By putting the sniffer "wand" into a dash vent, you can detect evap leaks that you could never see with dye.
My MAC sniffer will detect both R12 and R134a... I think most will.
You can easily find a good sniffer on eBay for less than 100 bucks if you're patient. You'll be amazed at how sensitive they are.
Dye is inexpensive but they charge outrageous prices for the UV lamp. Just go and buy a UV bulb (I've even seen them at Walmart) and put it in a drop light.
Hope this helps. KenP