They're much of a muchness, really. I've not been to Bank/Monument very recently, so am unaware of which, if any, parts of the complex are currently closed for rebuilding. But, under usual circumstances, the quickest way (as is increasingly the case at new TfL interchanges) from Monument to DLR is to not follow the signs for the DLR. Instead, go down the steps (signposted for the Northern line) at the west end of the District/Circle platforms, walk halfway along the Northern line platform, and then go down the stairs there to the DLR. Only catch: it's narrow in places, so gets congested, and consequently may be closed off during rush hours, leaving to you only the much longer "direct" route. But outside the peak, it'd probably be a quicker , (and, in rain, certainly drier) change than Tower Hill/Tower Gateway.