I was also wondering if the Ultra lows would be to low for me. I bought the 9er for the relatively relaxed seating position in the first place, not sure that I wanted to give that up. I was also thinking about just the Lows. The thing I noticed about the lows is the significant amount of pull back versus the ultra lows, streetfighters, and even the stock bars. I ended up compromising and getting the streetfighters, they are a little taller than the Ultra lows, but a little shorter than the stock bars. They are also a straighter bar, and a lot wider too.
I am about 5'9" and very glad that I got the streetfighters, they seem to be a perfect fit for me, and I really dig the crossbar. In the end it comes down to whatever fits YOU best. Under useful links is a link to renthal handle bar dimensions, look at the different dimensions shown, compare them to your stock bars, and pick what suits you best. I dont think you can go wrong, as the renthals are a big improvement over the stock bars anyway. Also look around here at pics of other peoples bikes with the different bars mounted on them, that should also give you an Idea.
Knees in the breeze!