Mushing glide vs forward slip sink rate
It really depends on the aircraft. Some have plenty of rudder authority and sideslip very well, others are better at stall onset with still plenty of elevator and aileron authority.

Now first to the term "mushing glide": This happens at very low speed when the airplane flies far at the backside of its envelope. Flow on the inner wing is already partly separated which causes the high drag. The nose is up because of the extreme angle of attack and the stick is pulled back close to its rear stop.

Not every airplane will be pleasant to fly: Some will show abrupt stalling and cannot be held in this mushing glide. Others have less stall margin between inner and outer wing and the pilot risks an inadvertent roll when the outer wing has an airfoil with poor stall characteristics (think 5-digit NACA series) and one side stalls first.

But there are also many airplanes which can be held easily in this attitude and with responsive ailerons and elevator, too. Those are the ones which lend themselves to a mushing glide. I would only fly this after extensive stall testing at altitude, calm winds and when I know exactly how the airplane reacts to control inputs.

On the other hand, if there is plenty of rudder authority to achieve a substantial sideslip angle in a sideslip, I would't think twice and pick this approach. Ending it is possible with little to none altitude loss whereas a mushing glide needs to be ended at enough altitude to pick up some speed for the flare. Visibility is better and the risk of stalling a lot smaller.