For my two cents I have also used both. The Dell 6224 I found buggy and unreliable, but then you get what you pay for. One particular bug meant the only way to get the switch working again was to power cycle!
The Ciscos will be more feature rich all round and far more reliable.
Having said that the Dells can shift more data across the network. However, don't believe what it says in the website as I think their figures are slightly massaged and from experience shouldn't get up to 95Mpps (it physically can't shift 95Mpps across 24 Gig ports, if you do your sums!)
This is just my personal experience and is open to debate.