The main reason for the Americans using Sail or fin mounted hydroplanes is their Artic strategy where they surface through the icecap and the sail breaks the ice. The Russians do not normally surface through the ice pack and prefer to stay beneath the ice.
Sail mounted hydroplanes are better in shallow running and at low speeds.
Hull mounted hydroplanes are better at depth and high speed which the Russians prefer.
One of the main advantages of sail mounted planes is the gear is external to the pressure hull whereas hull mounted planes are more complex to have the gear outside of the pressure hull.
Sail mounted planes do not interfere with the bow sonar arrays whereas bow mounted planes mean that design of the bow array is much more difficult

Finally the Americans are firm believers in “if it aint broke why fix it” 😂

The British operate below the Artic cap and surface through it as well and use hull mounted folding planes to prevent damage.
These give the same advantages as the Russian hull mounted planes with speed and depth

The main difference between bow and sail mounted planes is that bow mounted planes change the pitch of the hull whereas sail mounted planes enable diving while the hull remains level.

The British do surface in the ice and I have never heard of any problems with the hydroplanes - that does not mean there has not been !!

It is widely believed amongst sun dodgers that British sonar is far superior to American.

Never been in an Astute class only an S class for a short time.