Unfortunately, you will not successfully find a carrier that provides cheap/unlimited services in both countries, though most tier 1 mobile providers will roam for a less-than-fair price. However, I would look at obtaining a "Pay-as-you-go" SIM in Canada for the amount you think you'll need, then do the same when you enter the US. Sounds like you'll remain in the cities (and not head out into the rural areas surrounding), so you'll be safe getting the absolute cheapest service you can find. Major department stores or electronics retailers in either city like Best Buy will help.

If you're starting in the U.S., you should look into getting T-Mobile.  When I had friends visiting from Europe last year, I went to T-Mobile to get a stack of SIM cards for them and they just activated them online.

Not to sound like a salesperson or anything (I'm actually not on T-Mobile, though I probably would be if I were just visiting the States), but here's are the perks for T-Mobile for folks like yourself just visiting:

• It's month-to-month at a fairly low rate, starting at $40/month
• They have unlimited voice, text, and and data within the States.  4G LTE data is capped at a certain amount, depending on the plan you get.  As long as you're not streaming movies and uploading photos on it all the time, though, you should be fine.  Just use wifi whenever you can.  One of their new features is that you can stream music from certain music services without having it count against your 4G LTE limit
• They have plans with unlimited international data and text in many places (100% sure Canada's one of those places), which is great for travelers

I don't know if there's a Canadian version of T-Mobile, but if you get this in the States, you can use it in Canada as well.  T-Mobile has stores everywhere, and you can also order a SIM card to be mailed to where you're staying in the States (doubt they'll mail internationally though, so you can just get it when you arrive).  Here's the link to their individual plans for you to consider.

Simple Mobile is a good service and something you can get on Amazon as well. 

Also, the more popular companies like At&T and T-Mobile offer unlocked SIM options. All these services are around 40-60 dollars with voice and (substantial) data privileges. 

For the USA use Roam Mobility, see website for details. Unfortunately Canada does not have a similar service. All our CDN providers have Pay as you Go.

Tmobile has the best data plan world wide