There is no way to know without trying it.
Typically, ADSL is more dependent on the line length and quality of the wiring to the premises, but changing the ISP can make a difference, however only marginal.
Forget the marketing spiel (if it is the same in US as UK) where ISP's try and say about being faster than others - The actual connection speed that the modem connects to the ISP at is mainly dependent on the line length and quality of wiring but also technology used (ADSL, ADSL2, ADSL2+) and the annex of the connection (A, B, M etc.) (Scroll down to the grid of ADSL standards, I could not find direct link)
For example, a few years ago, the maximum speed of ADSL Max was 8Mb, the majority of people received 2-4Mb based on distance. A few ISP's took advantage of this and deployed ADSL2+ equipment capable of 24Mb and capping it to 8Mb meaning that nearly everyone they served could get a full 8Mb.
If the reseller is just a bulk reseller of Bell, it is unlikely that you will see any speed changes as it will be going through the same equipment. On the other hand, if they use different equipment such as their own DSLAMs, it is possible that they use either better equipment or different connection settings that can result in faster speeds.
Again, there is not really any way to know without actually biting the bullet and trying it.
(Also, remember, it is possible it can get worse by changing!)
On the other hand, if you are connecting at high speeds but are only pulling a fraction of the speed from actual websites then it is obviously just a contention / ISP issue and it will either resolve itself (less people on the line) or they will do something (purchase more bandwidth, better peering etc.). Moving ISP's can help in this situation.