The Canon 7D, 60D, 600/550D have all the same 18Mpix sensor, same metering system, and share a lot of the same technology and core firmware, so the still image quality on all of them should be basically the same. However as you go down from the top-of-the-line 7D, you progressively lose some features which may affect to the way you take pictures.
The image processor and frame memory size affects how fast you can shoot and how quickly the camera responds. Probably there is also an effect to video quality as well. The 7D is fast as a lightning and really good for fast action.
- 7D has dual DIGIC4 processors and the largest memory, it can shoot 8 frames per second, up to 58 JPG or 16 RAW in sequence
- 60D has single DIGIC4, and can shoot 5.3 fps
- 600/550D has smallest memory, and can shoot 3.7 fps up to 34 JPG or 6 RAW in sequence
More computing power and bigger memories do amount to higher power consumption. However, if you use the LCD a lot, or the internal flash a lot, then that consumes much more power than anything else.
The body structure:
- 7D has all magnesium alloy body with full environmental sealing, solid like a tank and even works in rain
- 60D has plastic + aluminium body, no seals
- 600/550D is mostly plastic body & no sealing, quite fragile but also lightweight, gets ruined in rain
The user interface:
- 7D has the most complete interface with dual dials, joystick, customization, 3 custom modes etc.
- 60D has reduced interface, no joystick, fewer buttons, limited customization, 1 custom mode
- 600/550 has a very simplified interface
The autofocus-system is different in all of these:
- 7D has 19 point all points cross-type autofocus with highly configurable & customizable settings, AF works really fast & is very realiable
- 60D has 9 point all points cross-type autofocus, limited configurations
- 600/550D has 9 point AF with only 1 cross-type point
The optical viewfinders are different:
- 7D has 100% 1.0x pentaprism, this means full and bright view of what you are shooting
- 60D has 96% 0.95x pentaprism
- 600/550D has 95% 0.87x pentamirror
Both 7D and 60D have 2-axis electronic spririt levels. 600/550D has only orientation sensor.
Connectivity:
-7D has USB2, HDMI, Video, N3 remote, external Microphone, PC-sync, IR-remote
-Others have no PC-sync, no IR-remote, and only support E3 wired remote
Accessories:
- battery grip available for all
- for 7D you can also get WFT-E5 wifi/remote controller grip
Canon Speedlite wireless flash support:
- 7D can act as wireless flash master with multigroup & channel support
- 60D can act as wireless flash master but no multigroup support
- 600D can act as wireless flash master but no support for channels or groups
- 550D has no wireless flash support
Rear view LCD:
- On 7D and 550D LCD is fixed
- On 60D and 600D LCD is articulated
Memory cards:
- 7D has CF-cards, others use SD/SDHC/SDXC
In conclusion: it is up to you to decide if the added features on the more expensive models are something you need and are willing to pay for. One thing to note though is the fact that any underwater housing with ports is going to cost you more or much more than the 7D body. Any lenses you have - or will eventually have - will cost you more than the 7D body. 