The ps2 modchip is more expensive if your going with the Messiah. However, if you just looking for a simple mod chip and dont mind the swaps its far cheaper.
You can buy a blank PIC chip for about $1.50 (less in bulk) and find a buddy/school/work with a PIC programmer and program it yourself with the free HEX online.
Buy a cheapo Interact memory card (16MB) and use that for your swap disc, gaining a mem card at the same time.
Or even the $40 GS would work, etc.
$41.50 for ps2.
Still cheaper than the $60 xbox chip.
...and yes, the systems are the same price so ya cant use that as a factor.