Hi,

It's probably helpful to understand and contexualize the different roles. 

• HR is a support function → they support the recruiting process and often are in charge of running the process
• While they can share input, typically the decision is made by ‘business’ or the business entity that is trying to hire for that role
• This is why very often HR is the one to do the first/initial rounds of ‘screening’, but the final rounds are always with ‘business’ themselves 
• Of course, the level of influence/input given by HR really depends / differs from company to company

In MBB, decisions are only made by the consulting team members who have conducted the interviews. A recruiting director / lead (who is always a consultant) may weigh in on difficult/borderline decisions, but again the important point isthat he is a consultant (and therefore the ‘business’ owner and not HR).

In a corporate situation, in my experience, if HR likes your CV (and is respected/trusted in the company), that's enough to get you infront of the hiring manager because frankly speaking the hiring manager does not have the time to screen CVs. 

If your goal is to secure the first interview directly, yes, it will definitely help alot if the hiring manager directly endorses your CV, but then it ultimately becomes a function of feasibility/practicality of connecting with someone who is likely very busy and whose day job is not screening/finding new candidates.

So in your situation, I would do both - network with both HR and Hiring Manager but I would be very tactful about networking with the Hiring Manager and probably place less expectation on that.

All the best!