Your question suggests there's only one dimension to it: form factor (size/shape of the added battery capacity); I think that's why Terry suggests you look into this a bit deeper. Mismatching batteries by age is not usually a good idea; also, you may find it easier to install and later replace a smaller battery (so buying 2 vs. 1 big one would seem to make sense) but it will take more effort to install solid battery boxes and cable up 2 batteries vs. 1 (which argues in the opposite direction). Gels produce varying performance on cruising boats; some last well and others die quickly, most likely due to charging regimens incompatible with the gel chemsitry. If there's a risk of an early failure, would you want it in a small/$$ battery or a larger/$$$ battery?
Take a good look at how your battery *charging* is managed as that will be key to good gel battery health. The controller for your wind gen and panels, if you have one, need to be compatible with that battery chemistry choice. Also, what about your 75 amp alternator? If it is externally regulated, you'll want to set that regulator up for gels. OTOH if it's not, the gels will never be properly charged by the main charging source on the boat, so that would be something else to consider.
Good luck on the project. For a cruising boat, this is one of those projects that is central to life afloat. You might want to read the relevant parts of Nigel Calder's _Boatowner's Mech & Elec Guide_ - it's indispensible.