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My 1.75 mm j-head (from China) is out of commission so I need to decide whether to buy a new 1.75 mm or 3 mm hot end.

lazerusrm has had excellent success w/ a 3 mm hot end on the Rostock Max printing at 100MM/sec 350MM/sec Travel:
http://forum.seemecnc.com/viewtopic.php ... s+3#p13015

The new Kossel on Kickstarter uses 1.75 mm filament because "... 1.75mm filament is easier to handle, easier to bend, and requires less driving torque, which helps us in the direct extruder drive configuration of this machine." http://www.kickstarter.com/projects/tts ... r?ref=live

So has anyone used both 1.75 and 3 mm w/ the new EZStruder on the Rostock Max and decided if one size will work better than the other? Is there a trend toward one size or the other in the industry?

You probably won't have enough torque with the easy struder to print 3mm filament.
It requires about 3x as much, you'll note that the Wades/Greg's designs are all about 3:1 ratios.

IMO there are trade offs in both directions, I use 1.75 because it's what I started with and I have a lot of filament.

This is a link I found back when I was looking at this same question for a printrbot jr. I went with 1.75mm.
http://fabbaloo.com/blog/2011/11/21/the ... ament.html

Thank you Polygonhell and Batteau62 for the great info. Looks like I'll be sticking w/ 1.75 mm.