Choice of frequency and protocol on RF links?

I am designing an RF link that is supposed to operate between an
embedded unit and a central control/collection unit (i.e. a PC with the
link hardware attached). The purpose of the link is to pass some data
(in form of strings), and then some control/session parameters. Here
are
the main features:
- Relatively low data speeds required
- Range of up to 20-30 meters (LOS, but occasional obstructions are
likely)
- Needs to handle multiple devices (i.e. units) in same environment
(i.e. hitting the same controller)

The RF link component/module will interface with some MC in the
embedded module.

I am wondering what choice of frequency and protocol to select:
- 433 MHz (good old AM/FM??!)
- 2.4GHz WiFi (802.11)
- 2.4GHz Zigbee

Main considerations are:
- Cost of component/module
- Ease of design/integration

I am aware that all the above options can and do work, but am looking
for the best fit vis-a-vis the above main criteria.

Suggestions and comments?
Thanks!

.

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