I recently visited the place, and I'd recommend staying at Joshimath, since the stay in Auli will be much costly. Joshimath is the nearest town hardly 10km away and has a ropeway to Auli.
If you want to go for budget trip, book your stay in Joshimath, as hotels are cheaper there than in Auli and you can find direct ropeway to Auli from Joshimath. If you want to go for luxury trip, you can book your stay in Joshimath, Auli in Hotel Cliff top Club. https://www.tripoto.com/auli/tour-packages