For me after playing a couple of campaigns with both I would definitely say Armenia. I really think they have one of the securest strategic starting positions on the map. To the North Scythia are too far away to pose any major threat at the beginning and if needs be you can stall them by blocking off the passes with forts. To the south and east Parthia and Seleucids are too busy fighting each other and Egypt to pose any initial threat. The Egyptians will leave you alone until either you or they take Antioch. The main threat is Pontus and here I found the best way was to make a dash with all the troops in Kotais the field army and the cataphracts in Artaxarta. Meantime I took the rest of the troops from Artaxarta and made a dash for Phraspa. This is important as its a good money spinner and when you take it the Parthians will leave you alone for the moment. I found that best way to proceed was advance west into asia minor eventually beating the seleucids, while keeping an eye on the parthians, enslaving most of the cities I conquered in order to upgrade cities and get access to cata archers. Then eventually knocking out Parthia and taking on the Egyptians. At that point Armenia has pretty decent armies, catas, cata archers, heavy spearmen and legonaries with a few horse archers and cretan mercs thrown in for good measure. When the time comes to fight Rome hopefully it will be pre Marius and the missile cavalry and catas will be able to hold their own against infantry dependent armies.

My Parthia campaigns in contrast were a disaster and I quit them in frustration in the end. I expanded into seleucid territory at first, while leaving armenia alone as I'd bigger fish to fry. Maybe that was a mistake as the Egyptians beat me to Dumatha and used it as a base to launch attacks against Susa and Seleucia. Meantime the Pontics expanded incredibly taking all of asia minor from tarsus westwards as well as armenia which they then used to attack my weakly held northern cities. Eventually they took hatra, splitting my empire in two leaving me with the antioch/sinope/cyprus strip under permanent siege from the Pontic/Egyptian alliance. How does Parthia survive early on? They have no infantry of note and it takes a while before you can get catas.

So I would alway go for armenia on the basis of the cata archers, better infantry and better starting position.