I started to play my first MP game and I found out following differences:
In SP mode and in Multiplayer/Hotseat mode orders works normally (for example Settler/Engineer work lasts as described in
Info: Settlers-Engineers (GL) ). But in
Multiplayer/Internet mode every order to a unit is executed 2 times.
This has effect to fortifying and Settler/Engineer work:
Fortifying don't last 2 turns, but only 1 (i.e. it works immediately); Settler cumulates 2 'works' the turn the order is given (but next turns it adds only 1 'work' to his cumulated 'work'):
For example grassland is reforested in 10 turns normally. But in MP mode this is done in 9 turns if you give the order and let the Settler work without wakening (first turn 2 'works' are made, then 1 'work' is added every turn). And the same work is done in 5 turns if the Settler is wakened and reordered every turn.
Why I post this in PBEM forum?
IMHO this behavior of MP/Internet mode makes the game less balanced than the SP mode. It causes a player has less strategic choices: especially roads everywhere and immediately are obligatory.
But you have the choice between MP/Internet mode and MP/Hotseat. Why don't you use Hotseat for PBEM?