I have a set up similar to the Party Star. It's pretty easy to fill off the bottling bucket and naturally carbonate or fill off the keg. I prefer the latter because it's just super convenient at the last minute. The system holds CO2 well, but you only get one batch per CO2 cartridge and they aren't cheap. Still, it's a good compromise between growlers and a full Corny keg in terms of capacity and portability.
The kegs I have (bought at the LHBS) have a bung on top for filling and the tap, but also have a gravity fed tap built into a bung on the side near the bottom. You could use that and not need CO2 cartridges, but I've never tried it.