I've done Roth twice, so has LJS and it's not for nothing that it is known as the 'best old race'. It is located very near to Nuernberg, on the Danube river, the swim wave starts are superb, the spectators are second to none and it is perfectly organized, as might be expected from a race that's been held for more than 20 years over the same course. There are also shorter events held and an IM relay too.
Having just been to Frankfurt I can honestly hold my hand over my heart and say the crowd support in Roth is far better. And it's cheaper! (by virtue of having surrendered official IM status - when the franchise contract ran out, the Roth organizers refused to pay a vastly increased fee to continue to carry the IM status. Good for them).