i'm currently using a Knog NERD. doesn't have heart rate or cadence, so i want to upgrade.

Polar CS500, comes with the wearlink strap and cadence.

Garmin. GPS, but tiny display.

and then there's the vaporous PedalBrain.
uses the iPhone, so that's one fewer device to carry.
adds a battery to the iphone.
biggest screen, most familiar interface maps
BUT - god knows when it's coming out, and the iPhone would be exposed to the elements/crashes, and there are no physical buttons.

so, what would you go with? is the GPS a compelling enough feature to go with a Garmin?

The better question would be if you can get a unit with GPS that is ANt+, Why would you even consider spending more money for a Polar that has neither.

And the display on a Garmin 500 has to be at least as big as on a Polar, and its user programable so you can vary the size and number of fields displayed.

with the 500, i can download someone else's route and upload it to the device?

^ Yes.

https://www8.garmin.com/manuals/Edge500_OwnersManual.pdf

Garmin 500

there is nothing better, except for the CAAD9

Have you been on polarpersonaltrainer.com yet? Seems like Polar is more heavy on training. So I guess it just depends on what you want. I was in this same situation recently. I think I want to focus more on training than riding with navigation.

well, here's my situation:

my daily training rides are on the same route. i'm finding that i push myself, then i slow down. so my heart rate is up and down for the whole ride. i'm thinking that having a HR/Cadence monitor will help me make sure i'm training effectively, consistently challenging myself. i don't need GPS for these rides.
but also.
my weekend routes are longer, and i am not enjoying cycle club rides. i would like to be able to download a cue sheet into the thing and lead a ride with friends to places i haven't been.

note, i like that the Edge doesn't need wires/wheel sensor for speed/etc.

so if the Edge 500 can do both...?

You can upload routes to the edge 500. If your wanting to pre map rides and lead your friends, the edge 500 probably is your better bet.

The 500 has a display of 1.17" W X 1.44" H
The 705 has a display of 1.37" W X 1.71" H
The resolution on the 705 is a bit finer than the 500.
The 705 also costs more, so if you don't need the added features, might as well save the money. I like my 705 for the most part, the color display, mapping.
You can compare the 2 here. https://buy.garmin.com/shop/compare....eProduct=10885
I don't think Garmin is or ever will be on top of the game as far as fitness goes, but I like what they have combined gps and fitness. More than anything, it records the data and I can look at it later on my pc. The gps is just a bonus.

as far as it not being the leader on fitness, i am in the same situation that most people are when they shop for bikes/bike stuff - i really won't know what i'm missing until i buy this thing, train with it, and see if i outgrow it.

my fitness goals are "get stronger, get leaner, go faster"
i'm not a triathelete that needs to monitor things on a very intense level.

i can believe that it's behind Polar, based on Polar's reputation, but in what way?

The focus of Garmin just seams to be on the gps end, it's what they do. For example, the calorie numbers on the 705 seem way off for most people. I don't even look at that, so I don't care. Some people do. I went from a $20 bike computer to the 705, so I don't have much to compare to. I don't regret the cost for the 705, and now you can find them online for under $400 shipped with cadence. That's a lot of money for some, but for me, you get a lot. We go camping to places I don't know. I can jump on my bike and just ride. The gps gets me back if I don't pay attention to where I went, and I can map out my rides if I want, be it ahead of time, or right on the unit. It has a lot of training features that I don't use, but others might. Once it is warmed up, it seems to track elevation fairly well. I say warmed up, as it sometimes takes a few minutes to find the proper starting elevation. I can't say what it is like without the detailed mapping, which costs extra, as I added it from the start. IF you do go that direction, I would suggest getting the map on cd and adding it to a memory card. IF you buy it on the memory card, you can't put it on your pc, or so I believe.

You can get a refurbed Garmin 705 bundle with cadence and HR sensors for $330 to $350 or so from reputable dealers like Newegg.com.