I'd be curious as to why you're looking at ZPL? Not that it is a weak fund or anything, it's just provincial bonds are certainly not something on everyone's radar.
I'm a big fan of 3 bond funds in particular, ZAG for an aggregate bond index, but then the BMO Long Corporate Bond ETF, which is ZLC, and the BMO Mid Corporate Bond ETF, which is ZCM.
ZAG is more government bonds than corporate, so the 3 of these funds can be purchased together depending on your overall risk tolerance to create a nice mix of fixed income. Obviously the longer the maturity the more volatile it will be pricing wise.
In terms of TDB, it is very similar to ZAG, but it is a bit higher yielding as it does have more long-term bonds in there increasing yield. It's a solid fund as well