It depends how long you're going to be away from a power socket for (and how big your backpack is) but I recently used the 10k one and got a couple of charges for an HTC One and a couple for an iPhone. Have a look and see what the battery capacity is for your specific devices and go from there but for a week at Glastonbury the 10k worked a charm for me. Hope that helps.
I have used mine for charging two smartphones and a tablet pc together and all seemed to work well. However, my purchase of the 10,000 mAhr item was largely because I wanted to carry it about whilst trekking on foot and therefore the smaller and lighter the better. If size and weight don't matter and price isn't an issue, then the larger capacity will obviously give longer charging capability. Hope that helps.