Yes, SIP would save a lot of money. The question about the "which provider" is a bit harder to answer, most of them do have advantages and disadvantages. For your specific case: you may want to consider getting a SIP provider in the country you will call the most (I am living in the US but make most international calls to Germany, so I have selected a US and a German provider). Since you are planning to use two devices, you also have to check closely on the terms of the providers. Up till a few month ago, Broadvoice only allowed one device using the same number, now you can pay for additional ones. Unfotunately there are quite a few providers. The other one I have good experience with is AXVoice. Both are US based. If you would ask for a favourite: My personal world wide favourite is Sipgate for many years - simply based on the best call quality no matter where I hooked up my phone, the great plan selection and decent pricing. The are however Europe based (and they do have an IPhone Client, unfortunately only for the first version IPhone, I am hoping for an update soon as well as for a US operation)
Last but not least: SIP on the IPhone: I have tried Fring as SIP client - surprisingly it worked with two limitations: 1. there was a pretty long delay and 2. it only worked on WIFI. Maybe not the best solution for a business call but for casual use OK.