I have done Nice twice, Roth once and racing there again this year, so for me they both have pros and cons. btw I'm French living in UK.
Nice: Registration is easy, it never fills in the first few days, rather in months. Swim is in a clear blue warm sea. Hilly 1 loop bike, 100% flat 4 loops run. The bike course is stunning, but not as hilly as the old Nice course (pre-Ironman). Forget PB there. However the scenery is beautiful. It is generally sunny and hot, especially on the run. The atmosphere is good with a lot of people along the run course, though it's a bit quieter toward the far end of the loop. On the bike some villages are very animated, and and other parts much quieter, but the views make up for it. Speaking English is a very small limiting factor as it's quite touristic and most restaurants, cafe etc will understand what you order. For tourism and family there is enough to do and visit (mountains, all the cote d'azur with Cannes, Monaco, etc), enjoy the sea. Travel and logistic of the race are extremely easy. The airport is at the turn around of the run course, accommodation is easy as there are numerous options in a large range of prices in walking distance from the start.
Roth: Registration is hard, full in few minutes. Swim is in a large canal in wave start. Less fighting than most swims, no issue of navigation. Bike is 2 loops quite flat (not as flat as many people think it is) course, but the road surface is very smooth, and of course it's a very fast course, very good potential for PB. Run is a one loop course, mostly along a canal. The atmosphere is unique, it's what makes this race really special (in addition to its history and PB potential). Spectactors can walk along most of the swim. They are crowding the bridge above the canal. You can actually see and hear encouragements during the swim. On the bike the Solarberg makes you feel you are doing the Tour de France. On the run there are places with again crazy atmosphere, but also much quieter portions along the canal. The finish is again amazing with a really good party. Logistics is a bit hard. Language is mostly not a problem. The best airport is Frankfurt, but it's some distance from Roth. Accommodations are limited, Roth being a small town with few hotels, but home stay is a possibility. Otherwise a lot of people stay in Nuremberg, but you need to rent a car. T1, T2 and finish are in different places. Although they bring your swim bag to T2, you still have to go from finish to T2. If you go alone, it's a bit of pain. If you have a crew able to drive your car, it's a lot easier.
So, which one is best for you? I don't know, they both are very good races and whichever you chose you should not be disappointed, but they are very different, both with a great history.
Edit: corrected 2 loops bike course in Roth, not 3
Francois-Xavier Li @FrancoisLi "We don't stop playing because we grow old; we grow old because we stop playing." George Bernard Shaw
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