Hi Ciara
Bwindi is tough, tougher than Mgahinga in my opinion and you will always be going up and down through thick vegetation. It can take 10-12 hours of hiking to get to the gorilla groups and back in the day (though sometimes 2-3 hours). Mgahinga is just up and then down and while it is steep, the trails are wide and clear although the altitude might sap a bit of energy.
Bwindi is pretty hard to get to but no where near as hard as the lonely planet makes it out to be. If you want to come straight from Kampala, I would get the Jaguar bus from Kampala to Kisoro, check in Kisoro at the UWA about permits, if they have some for Mgahinga, great, if not get one for Bwinid. To get to bwindi from Kisoro is around three hours for which you can take a motorbike (around 50,000 Ush return) or a private hire taxi (around 250,000Ush return). You could take a minibus from Kisoro to Kabale and jump off at the signpost to Bwindi forest but from here you would have to hitch into the park which might not be as hard as it sounds, particularly if you get there early as anyone else seeing the gorillas that morning will have to pass you in their cars (you would have to get to the bwindi turnoff at about 6am for this). The total cost of this would be around 9,000Ush one way, getting back might be easier as you could ride with whoever you trek the gorilas with but its a big risk as everyone might stay in the Bwindi lodges rather than ride back to town.
If you want to head to Fort Portal after the gorillas and Mgahinga is not available it might be best to do the run to kisoro, see what you want to see in Kisoro (volcanos, lake mutanda etc) see the gorillas and instead of getting the taxi/bike back to Kisoro after Bwindi, head on to Kabale which is the next big town. From Kabale you can easily get to Fort Portal in a day and there is a nice hostel called Home of Edarisa to stay in.
I am not sure companies will do a one day trek, the guides are provided by the UWA so all a company will do is get you a permit which the UWA can do in Kisoro anyway (or Kampala if you want to do things in advance). Most companies you can book through will probably sell you a package which includes accommodation and can be quite expensive. In my experience its far cheaper to turn up, stay somewhere and get a permit separately from the UWA. Unless you want to go on Christmas day i would say there is 100% chance of getting a permit for Bwindi in Kisoro, i was there for four months between December and March 2010/11 and never once saw a day when the permits were not available for the following day.
