If you want a polearm, Greatspear is a good choice because of its accuracy. It costs a feat (Weapon Proficiency: Greatspear), but it is worth it.
However there are only 4 good reasons to use a polearm for any class:
- Polearm Momentum for control. You should not have the 15 Dex for this, but even so, you do not have enough pushing and sliding powers.
- Polearm Gamble for control and damage. However no enemy in their right mind would approach the guy with the heavy armor and low damage. Unless you expect to spend your life in 3 squares wide dungeons, you will not benefit from it. Works best with Polearm Momentum anyway.
- You have bad defenses, but you still want to attack the target in melee. Basically this means you are a Leader.
- There are too many melee characters in your party, and you are unable to get adjacent. This is the land of party optimization. Basically, do not play a melee character in this case, if you must bring a defender create a Wizard|Swordmage hybrid.
A Defender's job is to limit the enemie's options. If you are adjacent, you do not have to attack the target of your Divine Challenge to keep up the mark, and more importantly, you can make an Opportunity Attack against the enemy if it moves away.
Reach is only good if you want to stay away, but as a Defender, you should not want that.
I would say you are better off with a Longsword and a Heavy Shield. Better defenses, less investment.