Whether you buy U100 or U120 mainly depends on you need HSPA internet connection on the go or not. You can use Smartone-Vodafone monthly plan, PCCW daily plan, or 3 prepaid plan. All can use 3.75G HSPA connection, very quick. If you put a 2G sim card from Peoples, the speed is only 150K.
If you only occasionally need on-the-go internet connection and don't want to pay for the extra for U120. You can buy U100, use a mobile phone with 3.5G (such as Nokia 6120C), connect via bluetooth, and still get HSPA speed. If you need it frequently, don't try to save the extra few hundred dollars and buy a U120.