What were the pros and cons of Canada west joining confederation?

There were numerous reasons why Canada West would want to join Confederation. In fact, it was probably the region which benefitted the most from the Canadian union. First of all, it was the dominant economy in the future nation. It had the most industry, agriculture, and railroads. From 1854-1864 the Reciprocity treaty with the US was exceptionally beneficial to Canada west. When the treaty was cancelled, however, Canada west faced a prolonged recession if it could not find new markets. As such, Confederation was seen as an opportunity to promote east-west trade within a future a Canada which Ontario would dominate.

Politically, Canada west had the largest population by far in the 1860s. The principal of "rep by pop" meant that this population would ensure that Ontario would control the future nation. Although at the Quebec Conference it was decided that the senate would be based on region, Canada West politicians were still confident that they would be the base of power in the new nation.

Furthermore, Confederation would open the way up for Canada gaining control of Rupert's Land. It was assumed (correctly) by the English Canadian expansionist movement that this future area would be dominated politically and socially by Ontario, thus overwhelming French Canada.

Pretty much all of these ideas were represented in the conditions that George Brown put forward for joining the Great Coaliton with John A. MacDonald and Cartier which allowed the movement to coalition to proceed.

There were also numerous other issues that were in play and benefitted the whole of Canada. Things such as the threat that the US posed, Britian's desire to get rid of the colonies, etc. were also of benefit.