For a 10g? Thats what I had...used that same regent fixture as well. I ended up selling it in the end...it just didnt cut it. Just go 150, and use a bluer bulb so you dont need actinics. Remember, bluer bulbs like 20,000Ks have 1/2 the output of 10,000Ks, so a 150 watt 20,000K is really like 70watts 10,000K...but the 20,000K can be run by itself and look good. The output of 14,000K and 20,000K 70watters is just plain dismal though. I couldnt even grow silver bar xenia at the top of the tank, nor low-light shrooms at the bottom of the tank with the 14,000K 70watt. Also, with the 150, you can use it on a larger tank should you decide to.
Even if you go 70watt, 10,000K, you will want supplimental actinic...so you are up over 100watts anyways...close to 150. So you might as well go 150 right off the bat.