You need to do a little more archeological digging still. You need to know the whole construction of the floor before you can decide how best to install something new. I'd take up that plywood piece in the kitchen and see what's under it. It's higher than the surrounding floor, so needs to go. You should also be able to see from there what subfloor is under the original kitchen hardwood. It's too bad the space under your entryway is finished, but I'd still do some exploring from below to see what's under there, and to figure out the fix to stop it from squeaking. Is it drywall below or a drop ceiling? I know it probably seems like a pain to tear out drywall, but if you cut out a clean patch, like 2' x the width of the joists, it'll be relatively simple to repair. Just make sure to cut it down the center of the joists so that there's room to screw the patch piece to it later. Cut the hole under the area that squeaks the most, if you can. Have someone walk on and watch the floor from below to see what's moving. It might be as simple as pounding in a couple shims from below, or face-nailing a loose board from above. You want to find out what's underneath the hardwood. It might be laid directly on the floor joists, or there might be 10" wide boards that the hardwood is nailed to. Another thought is to take out a board in the hallway, since that's not original and you want to replace it. See what the subfloor is there. I am a bit puzzled at why, when you've stripped the kitchen down to the original hardwood, it's still higher than the hallway floor. Is your current surface perhaps not the original kitchen hardwood? Peeling back the plywood area will help determine that. BTW, I haven't heard of that stop squeaking product you linked to, so can't offer any personal opinion. And don't pour self-leveling compound over top of hardwood. It needs to go over plywood.

When it comes to hardwood, trends are something to ignore! Why? Because hardwood is pretty much a lifetime purchase, and unless yours is damaged in some way, you're probably not going to replace it. So, light wood vs. dark wood, wide planks vs. narrow planks -- that stuff's all going to come and go. With that in mind -- as well as the very real concerns about cleaning dark wood -- I'll vote for a nice, neutral midtone every time. Not too much contrast, not too red, and (unless it's a beach house) not too pale. I think this is the wood that's most likely to give you good service and stand the test of time.

@Melissa Silva - Did you have the floors installed? How did it turn out? I just posted the same question over on the building a home forum. I am looking at Chelsea Plank and the store said I may have to worry about the the knots not being filled.

Geoffry_b is on it. Print off the NOMFA grading system. Take them with you when you go shopping. Have them beside you when you look at flooring samples. Notice the amount of variation (or no variation) that appeals to you. Then figure out the "other" things that come with that variation. Like knots. And allowable LENGTHS of boards that fit into that criteria. Here's an example: I'm someone who LOVES variation. It is referred to as 'movement'. Hickory is HIGHLY variable - especially in the "Select" grade (one step lower than "Clear". I also LOVE the look of "short ends" hardwood. The "short ends" refer to the average length of the plank....which vary from 18" (1.5ft) to 48" (4ft). Those are VERY short. That is one step DOWN (cheaper) than "Select". I also KNOW that "short ends" white oak has PLENTY of movement....just like hickory. The lower grade, short cut white oak planks have LOTS of variation and are SUPER cheap. That means I can "trade in" my pricier love of Hickory if I simply switch to the lower grade, lower priced, shorter planked white oak. So....My flooring choice (even without budget constraints) would work out to be: Select Grade White Oak in "short ends". I get EVERYTHING I want...in the LOOK that I want (and I know I will pay more for install because "shorts" are a pain in the A$$ to install - no one likes them). Now...on to your situation. Take your instructions for "grading" along with you and have them ENTERED INTO your CONTRACT. Your contract for your order/installation will include the words, "Grading of wood product will follow NOMFA Grading parameters set out in copy 'X' below". Then you list the grade you want as listed in the NOMFA criteria that most closely matches your esthetics. Excellent. Now pay the price for your selection and be secure in the knowledge you have done everything you can to ensure you will get the "floor you ordered" with the criteria you have accepted as "Nice".