Cat flea treatment

What is the best product for a cat that goes outside, I prefer the drops between shoulder blades.

Advantage or Frontline are the two that are trustworthy. Please do not use any of the cheap stuff from walmart, dollar store etc. they have a dangerous chemical in them that has caused seizures and permanent damage to pets. Sargent Bansect is particularly terrible and there is a change.org video and petition out there on that one. Go with Advantage or frontline. You can usually get Advantage cheaper at the vets or online. 

Which is better? Do you think I should get cat heartworm - flea treatment combo?

The 3 best are all different. Advantage, Frontline and Revolution.  Revolution is the only one to prevent heartworm and if you live in an area where that is an issue (many places especially the south) you need some type of heartworm preventative.

A comparison chart below:

http://www.1800petmeds.com/Advantage+II++to+Frontline+Plus+to+Revolution-compare50008.html