Things to consider:
1:What do you want to end up sailing? Do you have future asperations
2:Do you want to end up being a generally more versatile and skilled sailor?
3:How much space do you have to sail? Pretty much any body of water will be big enough for a small monohull like an RS Zest , Laser Pico, Sunfish or Topaz
How much space do you have for on land boat parking? (inflatable option)
4:Will you be sailing alone or with a friend?
5:Do you want to end up being a generally more versatile and skilled sailor?
6:Does easy speed appeal to you?
You will end up a better sailor -
better balance required
capsizes easier - the more you capsize the more you know how to manage the boat in that situation.
Better for solo sailing:
generally smaller and designed spefically for solo sailing
easier to right after capsize solo
lighter and easier to manage on land
easier to park
easier to transport
slower which means you don't need such a big space to sail on.
And - much easier, once you have learnt sailing on a mono to convert to a catamaran
less likely to capsize (assuming that you start in light winds)
not as much of your attention worrying about balance which means you can focus on course and sail setting
easy to stop and relax
learn trapezing much earlier in a cat sailing carreer
bigger platform means more space to take a friend
Easier to beach
One boat could suit the whole progression
Maybe better if you're are physically impaired
In the mono it's much more - are you going to be sailing solo or with a friend
There really is a massive choice with so many classes of monohull out there /
The most obvious single hande route is:
Zest - Laser - RS Aero - Musto Skiff - Foiling Moth
With cat sailing there are much fewer classes of boat to choose from,
The stability of the catamaran means that once you've got a good handle on the basics, you can go for pretty much anything - just as long as you don't try to sail in too much wind too soon.
The big factor with the more 'racing' classes is that they are significantly more fragile.
If it wasn't for that, you could pretty much learn on anything out there.
Cat - Hobie Getaway - Hobie 16 - F18 - Tornado - Foiling AClass
another massive bonus of monohull is that there are many many more places that offer sailing instruction in monohulls and if you were looking to buy a boat there are loads more monos out there than cats - to suit all budgets.
but, with the cat sailing, contrary to popular belief, it is easier to learn, as you don't have to worry about balancing the boat.
With either choice, starting off with light winds makes the biggest difference.