Both boats

The advantage of having two boats is that they often offer complimentary strengths. For example, during last night's shout the vessel in difficulty had drifted into shallow water near Old Harry which could well have made it difficult for the Mersey to approach given it's draft.
However, with the ILB available, we were able to use them to transfer people as necessary and pass the tow line whilst the Mersey waited further out to sea in deeper water.

Once the tow was attached, John stayed aboard and the ILB returned to station while the Mersey commenced a tow up into Poole harbour.

Another example is when we have a casualty on the shore under the cliffs, the ILB can get there faster and get into the shore, while the Mersey can provide the stable platform often needed to treat casualties upon.

So, we count ourselves lucky that we have two such fine, but different boats (as no doubt did last nights casualties).