Depends on alot of things. Condition of the motor/car. Km on the motor. 1 4 cylinder and 2 6 cylinders you can't really compare as they are different power wise, weight, suspension, different manifactors etc. They all can be reliable as each other if they are looked after and keept in check, if it's abused and used don't expect it to perform as you want it to!
On a different note i work at a nissan and mazda dealership and i had a customer come in after his service and say that we have to replace one of his engine mounts, cause the powersteering hose split and made it weak and tore it. I priced him up a new mount and hose and he spat it. Bloody crap blah blah im complaing to nissan and they are going to pay for it. I told him that they wouldn't as his car had been serviced 5 times in the last 140000 km and they would tell him to stick it.
So it depends how you are going to look after the car as to what will be more reliable and better off.