Manage them yourself particularly if you already have good tenants in place, I have always managed my 3 IP's (bar a 6 month period) and sometimes its a pain but I feel the agents just aren't worth the money. I have had a few dodgy tenants – one became a speed addict and turned on his girlfriend regularly, the other quit his job to deal drugs and messed my house up but we got through and had insurance. Ended up selling that house though cos despite it being nice seemed to attract these kind of people.
As for having a house vacant between tenants if you are organised  with your advertising it shouldn't be too much of problem. Its better to have a vacant house and wait for the right tenant than get someone in just to get rent. Always check references though and with their employer.

I have three rentals properties (that I manage myself) and at various stages all have been periodical. Depends whether you are happy as a landlord to have the expired term lease rolling over monthly or not and ditto for the tenant. Two (fantastic) tenants both prefer to enter into new lease agreements at the expiration which is fine. One advantage of the new lease  is if you want to change/add any lease terms. My last one had a rent increase and I also put in another clause that it would rise after 6 months so didn't have to worry about giving notice at the time.
Agents will also charge you more fees to do new leases so not in their interest to have one rolling over!
If they are good tenants periodicals are fine but if not having a lease in place with an end date protects you somewhat.