Mid-length board vs. Longboard

I've been surfing for about 1.5 years and started off on a 8'0 mini-longboard. Switched to a 6'6 Firewire ADDVance recently, it's been 4 months on it and I'm enjoying it, but it's definitely at the peak of my ability in terms of ease of use. I've been catching waves with it and even throwing small weak-ish bottom turns, but I now want to see how increasing glide and paddle power a bit again will feel after these last few months. Is it worth getting a mid-length high volume board or should I just get a full on longboard (8'6" / 9')? I know that the ADDVance is something that will probably least me years down the line and know that a mid-length board (7'0 - 7'6") may be something I won't keep. Thinking a longboard may be kept for very small waves. What do you guys think?
I'm 5'11, 165-170lbs and surf Northern California (SF/San Mateo). Usually go out when the waves are 7' and under.
Saw some of these mid-lenghts on CL and just wanted to get some thoughts.
http://sfbay.craigslist.org/eby/spo/5419037645.html
http://sfbay.craigslist.org/nby/spo/5417956833.html
http://sfbay.craigslist.org/scz/spo/5419416789.html
http://sfbay.craigslist.org/nby/spo/5422599711.html
Thanks!
I'm 5'11, 165-170lbs and surf Northern California (SF/San Mateo). Usually go out when the waves are 7' and under.
Saw some of these mid-lenghts on CL and just wanted to get some thoughts.
http://sfbay.craigslist.org/eby/spo/5419037645.html
http://sfbay.craigslist.org/nby/spo/5417956833.html
http://sfbay.craigslist.org/scz/spo/5419416789.html
http://sfbay.craigslist.org/nby/spo/5422599711.html
Thanks!