Hey all - co-founder of Plaid[0]. Congrats to Steve - great to see some innovation across the pond!
There were a bunch of questions about Plaid and the difference. The obvious one is that Teller is UK only and supports the top couple banks, Plaid is US only and supports thousands of financial institutions. If you need both UK and US coverage - since we both have pretty developer friendly APIs - it seems like a nice combo! Steve/Teller have also taken a bit of an antagonistic approach and has not worked with the banks - time will see if this proves successful, but we've taken the approach to work directly with the banks (as investors, clients, data-partners etc.).
Hope that helps and if you have any other questions/comments feel free to shoot me an email at william [at] plaid.com