Location of modulation and multiplexing in a GSM network

AT+CSCB is to "Select Cell Broadcast SMS Messages" - not to get a location: (datasheet, p. 70)

What you can try is explained here. First send the Test Command, AT+CSCB=?. You should get a response with a list of supported modes. Then you can try the Read Command, AT+CSCB? and you should get the current mode and so. At last you can try a Write Command, AT+CSCB=<mode> with <mode> one of the modes listed in the response to the Test Command. You can verify this change with the Read Command AT+CSCB?.

When you do want a location, try what PeterJ says in the comments on the question: send a AT+CENG=2. This sets the engineering mode (whatever that may be) and replies with all kinds of network information. Refer to the datasheet for an exact list, but it would include:

< mcc > : mobile country code
< mnc > : mobile network code
< bsic > : base station identification code
< lac > : location area code

With the bsic, you can use this to find your location. Other return variables may be useful as well.

My short answer is that there is no better or worse, it is a generic answer "that depends"

It is true that you will have a larger coverage with the lower frequency (assuming, the TX power is fixed). GSM is a cellular network which means that it consists of multiple small networks (cells) formed by BTSs (base transceiver station) which has an ability to serve maximum number of users. More dense areas (city centre, shopping malls, etc.) it is desired to have smaller cells (means smaller coverage) to be able to serve more users to provide a better performance (higher frequency, lower TX power of BTS). If it is a rural area, GSM operators may want to have a larger coverage to decrease their costs. Therefore, larger coverage does not mean that it is better or worse, it depends on where and why it is applied.

GSM900 has 4 power classes (39dBM to 29dBM) and GSM1800 has 3 power classes (30dBM to 36dBM). Transmit power of the mobile platforms are adaptive, closer to the BTS, lower TX power. If you have noticed it, the uplink of mobile devices are designated on lower frequency than the downlink in order to lower the power consumption on the mobile devices.

Higher frequency allows higher data-rate and most of all, it allows smaller antenna size which is important for the mobile platforms and easier to hide BTS's antennas in the public domains.