Yes weight.

A 6 weight standard WF line would not be heavy enough to load your rod. You probably need a 8, 9 or mabye even 10 weight line for single handed rods to properly load a 6 weight two-hander.

The tapers are also very different.

Sorry, Ill be a bit more specific.

The head of a Rio Windcutter Spey Line weight 365 grains for the 5/6 floating line and 455 grains for the 6/7/8 line. The Rio Selective Trout floating line for a 6 weight only weighs *edit*~230*edit* grains.

There are a ton of different tapers for different lines out there and they all match better to specific rod lengths, styles, makers, casting styles etc. Its a whole different ball game than single handed rods. I am no expert but there are other boards on the internet that can help you out a ton if there are no experts here able to help. I dont want to provide a link to another board because I thinks thats frowned upon (if it isnt some one let me know and Ill post it).

*Sorry, the 166 grains on their website is for the first 30 feet of line, the head is 42 feet.