Petronix is the cheapeast if you are buying new and payin retail. Sometimes requires slight mods to fit our 6's. There are 1000's of them out there running fine. There are other simalar conversions out there now for more money and less reputation. Keeps 99% stock look and parts. Works with existing coil if you want to upgrade in stages.
DS2 can be done cheap if you are willing to ebay and junk yard. Parts are easy to come by and still ford if you choose to run a ford module vs. the GM one. Some earlier blocks (170's?) require slight grinding to dizzy shaft to fit properly. DS2 can use the 74 Mav cap which looks like the points one and uses the same male ended wires. The standard cap for the DS2 uses female ended wires so you will have to buy new wires at the same time. DS2 can also use stock coil till you have the money to go further.
Both require re seting the timing and can benifit from opening up the spark plug gaps and better coils. I doubt you will notice any differnce in performance on a stock(ish) motor driven on the street between the two.
IIRC my DS2 setup cost around $200 buying new/ rebuilt parts and a new blaster coil, wires, and such.