Keep in mind that MiFi devices will often have several variants (like phones) which support different bands. This means that there's no guarantee that the E587 that Telus sells is the same as the one T-Mobile sells. According to T-Mobile, their E587 has UMTS 850/1700/2100. However, given that they listed the AWS band in an incorrect format, and it seems really odd (but not unheard of) that it would have 850 but not 1900, I'm not sure how much stock I'd put into their listing. There is also a risk that the unbranded one is a Euro version with no North American frequency bands.
Of course, they could all be the exact same device. Since carriers won't tell you for competitive reasons, and Huawei doesn't seem to be especially forthcoming on their website, it's pretty tough to tell. You could try digging through FCC filings for the T-Mobile variant and see what bands it contains, but even that is no sure thing. The only way to really know for sure is to test one.
According to Sierra, the 754S has 850/1900/2100. Assuming that is the only variant, that should work on Rogers/Bell/Telus, AT&T, and most European carriers, if you can find one unlocked. However, given the choice I would actually avoid LTE devices. Personally, I'm happy with HSPA+ speeds, and LTE technology is new and very rough on battery. Obviously it will vary from device to device, but HSPA devices will typically see much better battery life than LTE.