Riyadh or Jeddah

I would choose neither. Khobar or Dammam are better, quieter, and a causeway away from Bahrain where different rules apply.

You are right about the Bahrain thing, but as individual cities Riyadh maybe the best to offer u services and get u mixed with other cultures

Right! But it feels like a big prison after work, and weather in summer is really hot. Social activities are limited, and you need to have your own entertainment environment in your residence to survive (i.e home theatre, fast internet, swimming pool, paid TV subscription, ... etc) to survive. It much easier for boys to hang out, but no so easy for ladies as they cannot drive.

as I know social life is not so bad for western expats in Riyadh! they organise often partys in compound or in the embassies! u should only know the right persons.

three other significant differences:

1) jeddah is a lot more liberal, relative to riyadh ---> winner: jeddah
2) jeddah has very humid heat, and riyadh has dry heat ---> winner: riyadh
3) jeddah is dirty and has terrible roads, riyadh has cleaner and has far better roads ---> winner: riyadh

combine those with the 2 factors you mentioned, then weight each of the 5 factors according to what's important to you, and that will tell you where you should go.

I do a lot of scuba diving so for me Jeddah wins over Riyadh hands down. My job is based in Riyadh so it is easy for me as I don't have option to choose. Scuba diving will give you something to do and the compounds in Jeddah are quite fancy with nice private beaches. True Riyadh is a bit of a prison you can only hang out to go to the shopping mall and drink coffee with other guys to kill time

That's a lot of math haha, but I agree with u!