Looking for the right DSLR for my situation?

I'm in the market right now for a DSLR coming from a point and shoot. I don't want to upgrade in a year or two so I'd rather spend a little more money now, I just don't know enough to decide which one. I would like something light enough that I can put in the panniers on a road bike trip, also to take rapid pictures of my few month old niece to get that perfect shot of her using a prime lens, and lastly to take on vacation to new places. So far I'm thinking Nikon D5100 with one of the f/1.8G prime lenses, but also like the idea of the better rapid firing Pentax K5, but it is heavier yet smaller and I'm not familiar with their prime lenses. I also don't know if the K5's 7 shots/sec is needed over the D5100's 4 shots/sec. I also don't know if heavier is that much of a difference for carrying around. I like the K5 because of the image stabilization body, but maybe the VS from Nikon is good enough. I'm really open to either camera or something else. Please let me know if one of those is what I need, or if I should be swayed some other way.

I recommend going to the shows and events that camera manufacturers do across the country. The whole point of these shows is for you to have a play with all the equipment before you decide to buy anything. Plus you can watch live demonstrations of the latest equipment.

You'll find these events at various locations across the country such as hotels, camera shops and conference venues. Give Nikon a call and find out when the next one is near you.

I have a Pentax Kx (now discontinued) that I love. However I'm 70 an just re-entered the single lens reflex market. As far as Pentax prime lenses are concerned, if the lens are as good as their 50-200mm lens, they are superb! I also like the image stabilization body. But my likes aside you mentioned biking which I also like. I bought the Pentax case and as usual it is almost best to assemble you basic equipment first and then choose a case for everything. I'm not completely sold on the Pentax case.
But all this a side, My first reaction to your lengthy question is the space problem with bikes. An maybe you should take your time and also look into the new mirror-less cameras that are out. They cost a mint, but it they make more sense for your use, they can worth it to you.

I'm not that sure that the Pentax image stabilization is all that much of an advantage.

Both Nikon and Canon have in-body image stabilization in their compact cameras, but in-lens stabilization in their DSLRs.

So you have to ask yourself why both market leaders switch technologies with their DSLRs, as it is more expensive to do IS in the lens.

Perhaps the reason they did it is because it is better.

The K5 is the better camera overall. It has the very highly regarded image sensor found in the Nikon D7000 and Sony A580.It has the mag alloy body, and it has better weather sealing than any other dSLR in the same price range.

It's apples and oranges to compare in body IS with Pentax and Nikon VR. Nikon uses VR in their lenses. And while some might claim in lens is better, it can't be better if it's not there. Nikon makes zero VR prime lenses and it's not in every single zoom. So no VR for the 1.8.

So if you use a pentax, or even Sony alpha you have anti shake with every lens. If forced between these two, it's the K-5 but the lens selection is a bit limited when compared to the big three dSLR makers, and I've seen the prices on Pentax glass. Their basic lenses have prices that are in line with the other brand names, but their high end glass is very, very pricey.

A 50 f/1.4 prime for low light, and maybe their 18-135 for travel purposes would be nice. Take a look at Sony's A57 before you make a choice, it can do things that no Pentax, Canon or Nikon can do at any price. It doesn't make it better in all situations, but it might be a good fit for you.