I was faced with a similar dilemma at the start of last season in a similar position. Here was the compromise I made. I bought a senate, but I also got a senate that was too long according to their recommendations. I'm 5'11" and I was 185 at the time. I got a 69" senate. I was still doing long line, and I was skiing 28 mph pretty consistently. That ski did really well for me at 28 long line, then I went to 28 -15, then I went to 30 -15, then I went to 32 -15.
Toward the end of this summer I was getting pretty consistent with my 32 -15 runs, and I was almost never even bothering with 28 mph runs anymore. The large senate at 32 mph was starting too feel a touch large like it was just skipping across the water too much at that speed and wasn't getting in the water and controlling well enough. I found an amazing deal on a brand new, 2 year old vice-c, and went for it. So now I'm 175 lbs. on a 68" vice C that I skied for the first time last saturday. I feel like it was a good move so far as I think that the Vice C is controlling in the water better at 32. With the new Vice C, I have completely eliminated 28 mph, and now I am only skiing 30 and 32 on it. Hopefully soon I'll get the guts to go to 34 on it, and I think it will be a pretty good ski for that as well.
Anyway, long story short, where you are a touch smaller than I was at the time, I think if you grabbed a 68" senate, you would probably be a pretty happy camper. I was.