Recommendation on new $400-500 hybrid Analysis paralysis.  My wife says I can spend up to $500 on a first bike.  I don't want to spend it on the wrong stuff.  My intention is to use it as the "last mile" on public transit on commute/weekends, quick to the grocery, leisure ride, and perhaps even intentional exercise.  I've read tons of other posts on this site talking about what to look for in the abstract.  Now I need some concrete advice.  Which bike to buy?

At 6'4", 36.5 inseam, This calculator says my ideal mountain bike measurements are 21.5" seat tube and 180mm crank.  I don't know how that equates to a hybrid.  I think the larger frame size limits my options - bikes stores don't seem to carry these sizes.  So perhaps I have to order online?

Besides being the best value in all parts, certain luxuries I think I'd like are wheel splash guards and a rear rack as I might commute to work.  Can these be added to any bike?  Front suspension?  Not sure if I need it, but I know to want lockout if I do.  Aluminum vs Steel?  I've read that aluminum is less shock absorbing than steel, yet my known options below are all aluminum.

Here's what I'm looking at:

http://www.marinbikes.com/2011/bike_specs_compared.php?serialNum1584=1584&amp;serialNum1581=1581&amp;serialNum1580=1580&amp;serialNum1882=1882

http://www.performancebike.com/bikes/Product_10052_10551_1094212_-1_20000__400321

Are these any better than a Dick's Sporting Goods Diamondback brand hybrid?  

What others should I look at?

Thanks.
I've worked on a number of Marin bikes and they are impressive IMO.  Simple, straightforward design.

Here's the bottom line.  In this price range, regardless of what you get, you are looking at a Chinese-made frame built to "spec" (what the company wants).   Then, various major-manufacturer components are bolted on and the bike is shipped.

They are all really rather similar; variations in frame geometry are minimal, variations in component spec are also minimal.  It's common to throw on an "upgraded" rear derailleur as this catches the consumer's eye... Costs the manufacturer only a few bucks more.

Not that these are not perfectly fine bikes... The main thing you are looking for is fit, which you already recognize as important, and the best components you can get for your money.
Most hybrids are made with a frame that leans more to the "road" type, with a straighter top tube.  Most mountain bikes have a more compact design with a sloping top tube so that the rider can maneuver on the bike while off-road.
The old "standover height" or "inseam" height measurements date to when road bikes were built with the standard diamond frame with a flat top tube.
Modern bike design is a little more flexible.