Is there much difference between steel and duplicator? Apart from the price!
Depends on what steel, the "Prusa i3" is a open source RepRap platform, so a lot of company's are making them in all types of configurations and there are even more kits available.
the main differences will be the main board MKS/RAMPS/Melzi board they are using and what extruder they are using.
So, not all "Prusa i3" will be the same configuration, the good thing about the duplicator i3 is they have sold a shit tone of them over the last few months, so its really easy to find parts or print MODs specific to the duplicator i3 as it has a "standard" as in all of them have the same parts, over the trouble with Prusa i3 coming in assorts of configurations.
This one one o the quirks of 3D printers and open source, you start with a model like a RepRap that then branches off into Prusa models, then Prusa i3 then a Prusa i3 steel and then Duplicator i3, and at each of those branches you have different company changing bit here and there, so you can end up with two Prusa i3 printers that don't share a single components just a platform of design.
If you want to go with a kit, I would totally agree with
ax that a Prusa i3 variant is the best option, but you can often find lower specification Prusa i3 kits costing more then a ready build steel frame Duplicator i3.
Though there are many things I have even changed on my Duplicator i3, to get it printing the plastics I want and the way I want, as someone wanting to print something like PLA is going to need different MODs to someone want to print higher temp plastics like ABS or PET......or example cooling fan (PLA need to have cool air blown over it to harden it from the next layer, where as ABS/PET dont need cooling fans) and that where we get into the dark arts of heated bed materials, tapes, glues and sprays