I'm a Leupold guy and I don't own either of these. Having said that, both of these brands have good reputations as they great optic companies. Based on the specs that you have given, I would go with the Nikon for the following reasons:
1. That 50 mm objective lense is rather large in my opinion. Sure it lets more light in, but you will get all the light you need with the 40 mm. Also, the size of the lense is not the only factor in light tansmission.
2. The Nikon is substantially lighter than the Pentax. Even a few ounces starts to add up by the end of the day.
3. The Nikon has what looks like a great warranty.
4. It has more eye relief. (That's one of the important reasons I like Leupold.) This is a really important feature, especially when you have the power cranked up (which reduces eye relief) and are taking a shot from the prone position. Anyone who has ever had a "recoil kiss" from a scope to their eyebrow knows what I am talking about.
Based on these considerations, I would buy the Nikon.