Which is better SIM800 or SIM900

Low cost GSM / GPRS (SIM800) node

Yes, that's right, I meant DTR and CTS on the GPRSBee module, I wasn't fully aware that the SIM800 module has the similarly named pins, but that they are “generated” differently / not used on the GPRSBee module.

So far I have always assumed the GPRSBee-Lib, since DTR and CTS are mandatory. DTR switches the modem on (with the SIM900 version it was still a toggle switch, i.e. if it was on, DTR switched off, if it was off, it was switched on with DTR). The on / off status is signaled with CTS. But I just see on the beelogger page for the SIM800 module that the two pins are not absolutely necessary. Apparently the SIM800 is automatically “on” with an existing (or switched) power supply, i.e. we could do without these two pins / functions because we have / need a switched Vcc! Then, however, the SODAQ GPRSBee Lib would have to be adapted, or we would switch to the TinyGSM lib recommended by you with direct MQTT support.

In the parts list of the Beelogger GSM-Shield there are two resistors whose installation is not (yet) documented - I asked, but I assume that it is a voltage divider for the RX pin of the SIM800. Actually thought that a resistor-based voltage divider for such fast switching processes as with a modem is not the ideal solution, but it seems to work.

Also in the data sheet for the SIM800 module http://simcom.ee/documents/SIM800/SIM800_Hardware%20Design_V1.08.pdf are on page 30 (also page 30 as with your reference, just found only the v1.08) as a reference Design for the serial connection with a 3.3 V MC also recommended ordinary voltage divider with 1k and 5k6. So it can't be that bad.

TX level they don't shift at all. Why is the solution with the GPRSBee module so “complicated”, or can it also handle 5V via the serial?

Interestingly, Felix Rusu recommends a voltage divider for connecting an RFMxx to a “real” 5V Arduino (Uno) (3.3 V vs. 5 V) and I did the same with my Open Hive shields for the Arduino Yun and the run pretty well too.

For me that would mean that we can connect TX directly, RX with a voltage divider, DTR and CTS and switch the supply voltage directly from the Lipo via a transistor / Mosfet like the Beelogger or a P-channel as a direct high- Use side switch if it doesn't need too much when idling.

Good plan, or something wrong? ;-)