I would not say its a no brainer. It depends really.
Firstly, remember who the agent is working for, i.e. YOU. What do you mean he would not tell you the amount of the deposit. Is this for real? Have you any experience of renting a house or working with an agent before? Demand all the info you need it or else get rid of the agent.
Under what circumstances did the agent obtain a 'small' deposit. How small? Normal practice is a months rent, though sometimes perhaps a prospective tenant may give a lower amount but make up the difference within say a day (e.g ATM's have a max daily withdrawl amount). Find out when the tenant is paying the full deposit and then get written confirm or an email from agent stating they have the full deposit.
Obtain from the agent further details of the tenant, who they are, employers, etc etc. Find out when the rent is being paid, when the contract is being signed, etc.
Appreciate references may take a couple of days though any organised tenant should be able to produce them quickly. Be careful, a trick that (bad) tenants sometimes do is to produce references at weekends when it is harder to verify the reference and then seek the keys at the weekend. By the time the reference is checked its too late, the tenant has the keys.
Do not give the keys to the tenant until the reference is checked. That means you as owner also being happy with the reference, whether you place reliance on the agent to check it if you have used them before and trust them, or else you have to follow up on the reference yourself also. Don't easily take the agents word. (Given that the agent won't tell you the amount of the deposit my advice is satisfy yourself re the reference).
Re the couple across the road - apart from the landlord telling you they were excellent do you know in your own right that they are good. Not saying this is happening but you might be surprised to hear that a landlord may say good things to get rid of a bed tenant. You could always check with neighbours to verify if you don't know the couple well.
Assuming they are good, and it appears they are, question is whether you allow them to move in for €200 less. Personally if it was me, if I was able to satisfy myself with the new tenants, felt that there was a reasonable chance they might stay past a year (where were they before and why are they moving?), references checked out including employer references I would certainly consider leaving it with the new guys. No guarantee the couple across the road will stay longer and appears they are seeking a transfer.
Remember, renting is a business, weigh up the extra €200 per month against other factors and see which is the best overall decision, financial and otherwise.
Perhaps let the couple across the road know you have another tenant paying €200 more and split the difference if you decide to go with them? (ie they pay an extra €100, its very convenient for them also remember if they simply move across the road, friends, neighbourhood, etc).