most important is they have different chipset in the Sierra Wireless module. (GSM: MC8775 / HSPA: MC8790)
And number of antenna's (GSM:1 / HSPA: 2)
hence each needs its own firmware.
see this document: Configuring 3G Wireless WAN on Modular and Fixed ISRs (HWIC-3G-GSM, HWIC-3G-HSPA, PCEX-3G-HSPA-x)
http://www.cisco.com/en/US/customer/docs/routers/access/1800/1861/software/feature/guide/mrwlsgsm.html
It may also differ in being supported with wireless carrier partner per country.
http://www.cisco.com/en/US/prod/routers/networking_solutions_products_genericcontent0900aecd80601f7e.html
Regards,
Pieter
