You’re in an awesome position to make an informed decision. Talk with employees at your work who are currently represented by this union and see if they’re happy or not. Make sure you ask why. The quality of your local representatives is key. Being part of a large union has benefits like the institutional knowledge and ability to organize employees and help bargain the first contract, providing model contracts and resources for local reps, and then policing labor related laws and policies on a national level. But your local reps, especially in unions that have a more grass roots view point on how they should operate, are likely going to be doing all the day to day stuff and most, if not all, the negotiating.
Ask for a copy of the contract. Not sure in Canada, but in the US you would be covered by the expired contract’s mandatory terms until a new one is bargained. And the they usually don’t completely rewrite the contract, so any non-economic terms are likely to be similar if not the same in the new one. So you can see what kind of non-economic benefits your likely to have. Think lunch breaks, flexible work schedules, overtime procedures, vacation days etc.
You mentioned a pay scale that is more than yours but you don’t know what the new one will look like. Well, I’d ask for a copy regardless and compare the current pay scale and increases to what you have. Is it more? Because unless the company is doing poorly usually the pay rate isn’t going to go down with a new contract. You might not know how much more, maybe only a little. But if you would be doing better on their current pay table, then I would think the odds are likely that you will also do better under the new one.
You know what the union’s priorities for the new contract are, but what about managements? Ask the union if they know what changes management wants to make. That way you have a heads up of what might change in the contract.
I would also look at your local structure and the coworkers you would be joining. Is the current group the union represents much larger than your group they’re trying to add? If it’s a large group, are they predominantly one position or department or a mix of positions and departments? I’d be wary of being an add on to a large group of one position type. Because while they’re not supposed to, internal politics will lean towards the union caring more about the larger group if the two groups have different interests in a particular matter. The rationale being more people will be happy if we do x. If it’s an add on to a large group of one position type (different than your own), try to think of what the possible conflicts could be and if they matter to you.
Finally, I’ll note that a lot of people don’t think they’ll ever need the union until they do. People who’ve never had performance or discipline problems until three years later they get a nit picky supervisor sending memos on things like the font employees must use in emails (I wish this wasn’t a real example). Or people who have something bad happen in their personal life, unexpected death or illness in the family, and now are having performance issues or need to use a lot of leave and it’s being denied. But circling back, how helpful the union will be in these situations depends on how good your local reps are.