We have been in The Retreat onboard Summit many times and just recently onboard Silhouette.
Summit - Retreat Lounge there isn't a proper bar. Just a small area with maybe 2 high top tables.
The lounge itself is lovely, seating is a bit uncomfortable as the chair choices by the designer was a bit of a mistake. The Retreat Sundeck has access by a rather high staircase and also an elevator which doesn't always work. It does not have a bar, but servers are available. Two levels with a small dining and seating area and loungers on the lower level. Limited specialty dining. Ship is stunning since being Revolutionized in 2019.
Silhouette - Retreat Lounge is lovely with a full bar and seating. Retreat Sundeck 2 levels with a bar and overall the entire space is much larger than on Summit. Murano is definitely a highlight onboard Silhouette. And, the additional specialty dining is a bonus as well. Silhouette has also been Revolutionized and the updates are very nice.
Given the choice between the two our choice would be Summit as we've found the ship to be more welcoming and intimate. While Silhouette is stunning, we find that the Summit crew is amazing, service outstanding and for us, just an overall better experience. However, we will be back onboard Silhouette in April for a 10 day to once again enjoy the Retreat and all that it has to offer.