MicrostockGroup

simple answer to your question if money isn't issue I'd get the 580 as it is more powerful than the 430.
I am planing to buy either of the flashes but I am on a bit tight budged so I'd probably get the   430 but once again my first choice would be 580 and I personally wouldn't choose anything else but there are also some sigma flashes that get good reviews like sigma 530 dg etc. but if you are looking for a non TTL flash there are a lot more choices but like I said I'd personally stick with canon

good luck with your choice

I agree, don't buy the 220EX: you cannot tilt the head and this one is not very different than a basic on-camera flash.

Looking for an elegant lighting solution for my studio shots (objects, not people yet), I've recently purchased 1 580EX and 3 430EX as well as an ST-E2 flash trigger instead of studio strobes : this is an extremely portable and inexpensive lighting setup powerful enough for objects studio shots as well as 1-2 people shots.

The 580 as a main light, one 430 as the fill light and 2 430 for a nice and easy white background... it's all you need for semi-pro studio shots I would say.

For microstock, this is a setup I would recommend warmly  

This is the reason why I would recommend you to purchase either a 430EX or a 580EX: this may be the first element of your future lighting solution.

You need in this case one master flash (the 580 or the ST-E2 trigger) and 1-n slaves (the 430).

For your first flash, as you don't need a master-slave configuration yet, you may go for the 430 or the 580, the main difference being that the 580 has more lighting power, shorter recharge time and is more robust. But the 430 is a good choice too.

If you are going for a studio setup I would rather purchase studio strobes.  They aren't necesarily any more $$ than speedlights and work much better for studio use.  Only get speedlights if you are wanting to use them outside of studio work.

Can you elaborate on this point? I'm interested 

I decided to go with Speedlite after reading Strobist:Lighting 101 as I'm not currently shooting people but only still life and it seems to me that a hotshoes solution was a good one to start with even for 1-2 people shots.

I must say that I'm now quite impressed to see how easy and portable this setup is.

But I'm still interested in knowing which advantages offer studio strobes.

I understand that Speedlite are not powerful enough for groups of people, and you don't have a modelling light, but is there any other real advantage? Is it really a "much better" solution?

Because Speedlite also have huge advantages as soon as you shoot on location as they are a lot more portable (4 strobes are definitively not realy portables... only transportable) and you don't need an external power supply, only plenty of AA batteries.
 

Because I want to be able to put the 580 off-camera, typically on a stand with an umbrella or a softbox. This is a lot simpler as you then have a constant light on your subject while you move your camera to get the proper composition.

Otherwise the 580 as a master is even more powerful as the ST-E2 because the latter can handle only two groups of flashes while the 580 can handle three groups (main - fill - background).

From http://www.bobatkins.com/photography/eosfaq/ex_speedlites.html

The 430EX II features the same basic appearance and layout as the 430EX, but adds a metal shoe with the same locking mechanism as the Speedlite 580EX II and 1/3 EV step output control in manual mode. The 430EX II can also be fully configured via the rear LCD menus of recent Canon digital SLRs as well as from it's own LCD control panel. The recycling time of the 430EX II is 20% faster than the 430EX and recycling is much quieter. Like the 430EX, the 403EX II can act as a wireless slave, but not as a wireless controller.

I never used studio strobes, but I know that you are not limited with camera flashes to the ceiling to get the soft light effect. You can use a softbox or an umbrella with normal camera flashes. But you are right you do not have the same power -> less depth of field options at low iso. I have not got a canon, but a sigma EF-500 DG Super. Its powerful and cheap, but also cheaply built.
For off camera flash I got an old Nikon SB 24. With a cable its probably one of the cheapest off-camera flash options for canon