In a straight comparison of features:
Unit Support: Republic 1/town, 3/city, 4/metro, 2gpt above the limit; Democracy: no free support, 1gpt per unit.
Republic wins hands down, especially once you get 30-40 CITIES (size 7+). With no MPs, chances are you'll be paying very little to support your army - and the break-even point with Democracy is going to be very, very, high.
Corruption: Democracy just edges out Republic - but only just. It's hardly noticable.
Worker Rate: Democracy gets a 50% bonus to the Worker rate, so wins this round easily.
War Weariness: Republic has much more manageable WW than Democracy, so Republic wins this one.
Commerce Bonus: Both get the same bonus, so a draw.
Availability: To be able to research Republic, you don't need to research any useless techs, you can sometimes get it for free, and it's available ver early in the game (if you pull of a successful slingshot, nearly always before 1000BC). Democracy has to be researched, you need a useless tech to get to it (Printing Press), and comes along quite late in the game. The big killer, of course, is that unless you're a Religious Civ, revolting to Democracy takes up another x turns of Anarchy.
Republic wins 3-2 (and the 2 points for Democracy, namely corruption and worker rate, really are the most minor aspects).