I have had two fire hoops -
I am 5'9 which is taller than both of you. I have been fire hooping since December - probably getting in a fire session once or twice a month. My first fire hoop was a 40 inch from spinsterz. That hoop is heavier because it breaks down into three parts and clicks together. I had to tape over the plugs because they bugged me and taping made the hoop feel more stable when spinning considering the torque that builds up with the rubberized flex spines.
6 spines - total length 8 inches w/ approx 3/4 inch threaded and screwed into the hoop. Large wicks -coiled kevlar 1.5 inch webbing secured with a wire and some sewing along the outer seam. It worked but didn't feel perfect. Maybe because I was using a much smaller hoop for everything other than fire and lighter too. I couldn't do many of the off body moves I do with the smaller non fire hoops. I spun this hoop about 12 times before looking at others hoops and shopping the web.
My next hoop was made by Holistic Hoops - custom by Kandace. 38 inch/one piece of tubing w/ one connector.
5 spines - 6 inch total w/ approx 3/4 inch threaded into the tubing like the spinsterz. The wicks are smaller and kevlar wrapping cotton wadding.
I have spun this hoop now on three occasions. I love the spacing of the 5 wicks and with the smaller wick head size, the flames are not too intense with the shorter spine. The spines flex with a large spring and this is not covered. With the spinsterz flex you have an area that doesn't get hot like the spring spine can at the end of a spin. That is my only issue -(I add here that the wicks burn about 2 min longer than the spinsterz...2 min is long when you are working fire)
. I love this hoop and currently rate it as 99% heavenly.
So, you see, there are all kinds of other factors to consider beyond the spine length. I think the 6.5 would be sweet, but then that's just me. Let us know what you end up with and how it goes (glows)