We loved both Le Sirenuse and ISP, but ISP was by far the preference. I will post a full review of Le Sirenuse in due course.
We chose two hotels in Positano because Le Sirenuse was a bucket list hotel for me and I wanted to get the experience of being in town as well as the more relaxing, private style offered my ISP. We chose to spend the majority of our stay in ISP and I'm glad we did for the following reasons:
1. Arrival - the lobby in ISP was calm and serene with only a very few other guests. We were immediately taken outside for a complimentary cocktail as our room wasn't ready and then went for lunch at Zass.
By contrast Le Sirenuse had a lobby full of people when we arrived and we had to wait to be seen. Again the room wasn't ready (but again this was expected) but we weren't given a welcome drink, not even water. We headed to the the hotel restaurant as planned but the waiter said rather pointedly we couldn't sit on the pool terrace as that was just for guests and the restaurant was full. When we said we were guests but just didn't have our room yet, he was all smiles and gave us a fantastic seat.
2. Privacy - The terrace at ISP is beautifully private and a relaxing place to drink in the evening. Zass welcomes outside guests but Carlino is guests only. No where ever felt overly busy or crowded. The pool is totally separate from the restaurant and has it's own bar so there are no outside guests. There is a private beach.
At Le Sirenuse the restaurant welcomes outside guests and it is located right by the pool. Even though the pool is guests only, it still felt busy because of the proximity to the restaurant. There is no private beach, it's about two minutes walk from Positano beach which is crowded and touristy. The bar at Le Sirenuse is much more busy and buzzy which we do enjoy but is a matter of taste as to whether you want this. Perhaps more a bar to visit than one in the hotel you're staying in.
3. Value for money. Neither hotel is good value for money but ISP is margainly better as the rooms are bigger and less expensive. I would say LS had more well appointed rooms but they were small and felt poorly planned. Not every room has a sea view. The bathroom was tiny not even a separate shower. By room square footage LS is the most expensive hotel I've ever stayed in. Velaa gets the prize for overall most expensive but we had a huge over water villa with private pool rather than slightly pokey room and small terrace. We've stayed in more expensive hotels such as Singita, Mnemba etc but these were all inclusive, LS includes no extras.
Food-wise the hotels were equal in quality. We tried both restaurants at the ISP and they were fantastic. We didn't eat in the champagne bar at Le Sirenuse but La Sponda was fantastic. Both La Sponda and Zass have a Michelin star. We missed out on Francos as I assumed it was the other bar in the hotel, no one told us it was located separately! As you can imagine this annoyed me considering how much we'd spent on the room.
One advantage of LS was the view, one of the most spectacular I've ever seen! Apart from the minor niggles mentioned service was fairly equal at the hotels, very warm and helpful. Many staff members have been there half their lives!
I realise this seems like a bit of a bashing of Le Sirenuse. We genuinely loved both but we stayed at LS second and it was a little disappointing after ISP. If doing both definitely stay at LS first and leave the best for last. If you don't have time to do both stay at ISP and have dinner at LS, they are around 15 minutes drive apart.