Unless there has been a change, the Star Alliance website RTW tickets always come out on Lufthansa ticket stock. If you are looking for United mileage you may earn a fair bit more if the ticket is issued by UA. The ticket stock is on the carrier with the first long haul overseas segment. If you call United they can also issue the ticket themselves provided they have the first long haul segment. It would definitely be best to use the Star Alliance online tool to plan what you want, check the mileage and fare, and then provide this to your travel agency or airline (especially if you are trying to maximize mileage earning). Otherwise LH has always given me great service and I would pick them to issue the ticket over anyone except United.