Why restrict yourself to those two -- Toshiba have some nice machines, Sony make the best looking outside of Apple. Asus. The truth is each sells a range of machines -- and incidentally don't actually make them but buy them in -- and some in each range are inevitably better than others.
Lenovo apparently make their own machines -- and I've never heard anything against them. So. on Dell vs Lenovo, you might consider that a vote for Lenovo.
Dell machines vary in quality by model because they're made by a range of suppliers. What I will say is that their Service Tag ID (which identifies the spec of the machine quite precisely) is incredibly helpful when you need to use their support site to find drivers etc