Far-away D-link AP instead of 2 meters away Airport Extreme?

Question

In my setup:
Airport extreme channel 1 in office
D-link AP channel 6 in garage (25 meters away from office)
My neighbours Netgear: channel 13
Both of my Acces Points have same SSID, same WPA, different channels (as above). Of course, my neighbours SSID and WEP is different.
Any mac computer chooses to connect to D-link, even in my office. Only iPads and iPhones connect without problems to airport express, and switch seamlessly from one to another. Computers have a lot of difficulties connecting to airport express. Without the D-link, 8 times to 10 there is no wifi connection, even in the office.
Airtunes however, always work. Even (and mostly) from my lan-connected imac. When i enable airport on that imac (in office), he connects to d-link. In rooms even further away from the d-link (say 35-40 meters), but within 10 meters from airport express, there is no connection.
If that is an indicator of how good wifi products are, apple has a lot of work to do on the main subject of an access point: being accessed!
This is a problem to me for 6-8 months now. I have tried really everything, going from WDS, additional time capsule (I already had before the express, but now with disabled wifi due to bad location), playing around with channels, ssid's, wpa's,...
Ofcourse, all of my software and firmware is up to date.
The airport express is 802.11n
The time capsule is first generation
The d-link is 'extreme-G'
The macbook having trouble connecting to AE is a pro intel dual core 3 years old, osx lion
A second macbook having trouble connecting to AE is the white unibody intel dual core 2-3 years old, osx snow leopard
The imac having trouble connecting to AE is a 17" G5, 5-6 years old, osx leopard.
I don't think there is a solution for the problem, I rather think airport express is just meant for airtunes and printer sharing. The access point capabilities are just bad.
But hey, Apple, as I have a lot of your products: Proof yourself!

Answer 1

First of all, you're mixing D-link and Apple devices on your network. It's not that this doesn't work, but it means you can't count oun how things are going to work.  You also don't say if all of the devices are using the same wifi protocol? 802.11 a/b/g or n?? 
Depending on the design, setup, and capability of the wireless client, it's going to go thru a certain routine to try and select the 'BEST' wifi signal and transfer rate. Older devices that are not up the -n standard won't connect to to a wireless access point that doesn't support it, or isn't backwards compatible.  Then, downselecting to the older protocol may result in it finding a stronger signal from the device of a different manufacturer that is actually further away.
I have personally mixed d-link hardware with apple hardware and had to spend a lot of times with channel and mode settings to get things working the way I wanted.  When I was finally able to switch to an all Apple device network (4 extreme's and 3 expresses) things were a whole lot easier to setup and configure and only then, did things behave the way I exepected, with the clients picking up the strongest signal and then moving from room to room and switching to the strongest signal in the room at the time.
You're mixing wifi clients of different ages, with different 802.11 capabiliies with wifi devices from differen manufacturers, it's possible to do, but NOT EASY. Don't blame apple it's really not their fault.