THe wholel Dell VS IBM arguement is more of a service/support, quality, and durability arguement rather than cost. The IBM "Think Express" models tend to be more price competitive with DELL, then SKU'd not labelled "Thnk Express".
On the whole, I've found IBM quality, and durability to be far higher than that of the Dells.... I don't care how many times Dell runs the commercial of them abusing their notebooks.... all Vendors do that.
IBM uses more Titanium/Magnesium Composite in their TP's than Dell in theirs... They also have the Active protection system, which I feel saved my data's ass about four weeks ago. I've also been running their Rescue and Recovery with RRPC.... and It's cool as anything I've seen from any major vendor, and Dell doesn't have it.
Yeah, I'm a big IBM fan, but that's due to having learned the lessons from the Dells I deal with in my environment.