@jamesUS: What do you mean by a "long" and a "short" antenna? If you use an antenna of 8.6cm long, it is a quarter wavelength at 868MHz. I regularly use a piece of wire of 8.6cm, and it performs very well. We are talking about >50km that I reached with this.
Of course the antenna on the gateway also has an influence. I will try to summarise some of my findings on the wiki some stage when I have time. But my rule of thumb is basically:
A high gain antenna on the gateway, installed at a high and open location.
A very low gain antenna (min 2.15dBi) on the mobile node.
If you have a fixed node, you can use a directional antenna on your node pointing to the gateway, but keep in mind that the ISM regulation does not allow you to transmit your than 14dBm (25mW) EIRP. The antenna's gain therfore affects this. For example if you have a 2.15dBi antenna, your radio may only give out 14 - 2.15 = 11.85dBm.
An important reason you want a low gain (as close to omnidirectional as possible) antenna on a mobile node is because the node will be moving around, and you can not guarantee that the gateway is in the direction of the antenna's highest gain. Maybe a little unclear, but I'll try explaining with pictures on the wiki a little later.