Northern route is a good choice because of the concentration of both employment and residents. The Kanata North BRT was one of the highest-ranked BRT projects for this reason, which is why it was one of only two projects to survive the 2013 TMP's BRT cuts.
Here's my idea for a low-cost use of the northern route:
The yellow line is a cheap, Trillium-line style diesel route on the existing railway.. diverging from it at the end to cut through the woods to the new Innovation Park & Ride.
To connect it to the LRT network, the Bayshore spur is extended south to a new junction at Queensway Carleton Hospital, while the Baseline spur is extended south to a new junction at the railway line north of Knoxdale, with a station at Meadowlands/Tallwood thrown in. The route along the 417 to Kanata centre can remain as BRT.
The extension from Baseline to Knoxdale would be very cheap, because the city is already planning to build service tracks on that route to reach a new LRT yard in the area (its location is marked with a diamond my map), so the only extra costs are building stations at Knoxdale and Meadowlands, which as surface stations wouldn't be too expensive. That yard could also double as the yard for the new Kanata line.
The extension from Bayshore to QCH would be somewhat pricey as it would involve having to go over or under much of the 416/417 junction.. however, it's still a short route so it can't be that bad.
As a diesel route, the Kanata line itself would be fairly cheap to construct.
What's more--this new Kanata line could allow the city to cancel the March Road BRT, which is currently funded in the plans at a cost of $110M.. 100% paid for by the city. This new proposal could be split 1/3 between city, province, and federal government. So the $110M the city had earmarked for the March BRT can be redirected to form part of the city's share of this new project.
The original O-Train line cost about $80M, if I remember correctly; adjusted for inflation that would be about $110M today.. my proposed Kanata diesel line is about twice as long, so say maybe $250M. The Baseline->Knoxdale extension would only require building two new surface stations, at a cost of maybe $40M. The Bayshore->QCH connection might cost, say, $100M due to the high cost of running the line over/under the freeway junction... so, with my back-of-napkin analysis, the project would cost maybe $400M.. let's go up to $500M because everything is more expensive in the end. So that's about $165M to be split between city, feds, and province... $110M of which can be found by cancelling the March BRT. So we really only need another $55M of city funds, shouldn't be too hard to scrounge together.
Eventually, the yellow line could be extended to Greenboro to provide a crosstown transit connection, and connectivity to areas like Merivale Road and the Colonnade Business Park that are currently poorly served by transit. However, this would be a lot more expensive because of the need to not interfere with VIA traffic on the bridge over the river.