I have two 8' poles on the back, but I very rarely wish I had 10' poles. It's weird, I used to always wish I had 10' poles, but in the past few years I've just gotten used to knowing that I have only 8' poles I guess. I just know that I need to to be in about 6 1/2 of water or less; 7 feet if it's calm out. Don't get me wrong, I'd take 10' poles in a heart beat. I just don't need em' right now.
I am glad that I have 2 poles though! I just always hear people say that they wish they had 2 instead of just one. One pole is fine, nothing really wrong with it. You just get a better hold with 2, and your boat won't swing/pivot as much in the wind.
If you can swing it, get 2. You'll be glad you did in the long run! Also, if your budget allows it, get the Blades. The Pro Series II's are great as well! But I'm so glad I have Blades! My buddy has 2 Sportsman Series poles on a 19ft Legend, and in a stiff wind or a wake that's 3ft or bigger, his poles will come off the bottom as the wake comes through. They also only hold well in about 5ft of water.
All in all, you'll be happy to have a Power Pole no matter what. They really are a great tool to have on your boat, and I will always have them on my boat. There's basically zero maintenance that comes along with them, and let me tell you, they are DURABLE! I know you'll pleased to own one, no matter what model or size.
"If people concentrated on the really important things in life, there'd be a shortage of fishing poles." - Doug Larson