Hi Soggy and welcome to the odd world of Subs
Glad someone else on here is getting into subs, they seem to be getting a little more popular these days.
As Bunker says, the Robbe and Engel are different animals altogether. I started with a Robbe U47, and went onto a Metcalf mouldings Royal Navy "T" class sub, and eventually an Engel Typhoon.
I suppose it comes down to one 2 questions, do you want a "dedicated" submarine, ie the Engels or a surface boat that can dive occasionally.
I mean the Robbe boats are Dynamic divers as Bunker says, which dive quickley under the water for a short period of time, or a proper sub that can stay under the drink for as long as you want.
And secondly how much can you justify in spending. Or how much your other half will stand for before divorce procedings get under way !
The Robbe boats are good, great to start with and get some experience, and when there done they look good. But when there put alongside an Engel there is no comparison. But they cost a lot less. You can upgrade them with a brass etching set for upper works etc, and you can add Engel Diving tanks to make it a true "static" diver. But the cost soon mounts up.
Where as the Engel is a complete kit, ( I use kit in the loose sense, they still need major adjustments ) and when its done they are really impressive and are worth the money. They can be tricky. Cutting the holes in the deck is a real pain, and a thorn in the side but its not too hard, and if you cock it up theres always fillers !
I would say if you want to start, get some experience get the Robbe, but if you like a challenge, ( not insurmountable either ) get the Engel. IF you get stuck there about 5 of us on here that are sub nuts. Check out my thread, and Manksters, Solos which is an Engel U boat and ADJ who has a typhoon as well.
Go for it, you wont regret it that often !
Paul