The highest data speed achievable for HSDPA or 3.5G is 7.2Mbps. I'm using a Huawei Modem E220 which supports speed up to 3.6Mbps and LG KS20 3.5G phone also supports maximum speed of 3.6Mbps. Since my mobile broadband plan is only 1Mbps, there is no point to pay more for a device which can support 7.2Mbps.
Similarly, if you've a iPhone 3G which supports speed up to 1.4Mbps, it is not worthwhile to subscribe to a mobile broadband plan with speed greater than 1Mbps. The data bandwidth is limited either by the device or the mobile broadband plan.