As I read I should go for 3mm filament extruders. But all of them seem to be extremly expensive. What reliable extruder for 3mm filament would you suggest?
Or should I better go for an 1.75mm extruder?
Honestly it’s all opinion here. What nozzle size are you planning on working with? If it’s a .5, either is fine. If you’re using a .35 or smaller, go for 1.75mm filament as there is less back-pressure.
Accoording to your spark a greater gear ratio for 3mm filament would make more sense to reduce backpreasure for smaller nozzles?
I’ve read that most quality filament would be 3mm. But I’m not sure of this statement…i’ve read that some days ago 
Quite the opposite. 1.75mm filament is almost always made specifically for 3D printers. A lot of the time “plastic welding rod” is 3mm, and of varying quality. If you buy cheap 3mm plastic, you’re much more likely to get crazy stuff like ball bearings in the plastic than you are buying cheap 1.75mm filament.
And… my “spark”? What is that?
J-head nozzle. I have a variety of different sizes and a quick change setup. works great.
@ThantiK I ment the backpreassure with your spark
So I should go for an 1.75 extruder. What’s the advantage of J-head nozzles? Didn’t hear that before…
1.75 is the standard
@Bertier_Luyt There is no standard, both have their merits. @Michael_Weber j head nozzle won’t get you to 300degrees, for those temperatures you need an all metal hot end. The current favourite (apparent community consensus) is the E3D one, I have one myself and it is a definite step up from the Jhead I have.
With respect to the extruder (cold end) they are normally agnostic of filament diameter. If you want an off the shelf one, I’ve heard good things about this: http://shop.seemecnc.com/EZStruder-Cold-End-Kit-70780.htm
Otherwise, if that is too pricey, there are stacks of printed extruders on Thingiverse, I 'd hazard a guess that the most ubiquitous is Greg’s Wade Reloaded version: http://www.thingiverse.com/thing:18379
I tried 1.75, went back to 3mm.
The main reason for 1.75mm is you can use a less powerful extruder motor as it is easier to push through the hot end. you basically get a mechanical advantage by running a weaker motor for a longer time for the same volume printed. this is why many ‘mini’ designs use 1.75mm. If you have a full sized one, might as well do 3mm.
1.75 gives you more precise control over volume extruded than 3mm does.
That’s a good point. Which torque should I go for?
There are a whole bunch of us at work who’ve fallen victim to the temptation of 3d printers. Among us, we’ve done 3mm and 1.75. Seems you can get good prints with either, but very important is that you get good, consistant-diameter filament. Cheap stuff sometimes has a lot of inconsistancy in diameter… Pay a bit more for stuff that guarantees consistency.