Pure Play AM vs Banks

The biggest difference is that when you are at a bank - you work for the bank. That might seem obvious but at the high level, it creates the most distinction in the strategic outlook of the AM unit, how things are managed, influences on upper management, career/compensation/job security.

For some AM units, they are more 'captive' to the bank's clients - i.e. they are fundamentally an outlet for additional assets the bank doesn't, can't or won't take on their balance sheet. They may also create products or strategies that cater to specific clients - i.e. corporations who bank with XYZ may want a daily liquidity option, maybe they'll have a money fund that offers them an option. Maybe it's a set of funds/strategies for institutional clients that are managed a certain way vs. others that are out there (i.e. core fixed income that's more, or less, conservative and an easy option for clients of the bank). Wells was another example, when they had the asset cap they tended to push assets to their AM arm as it wasn't subject to that since they weren't balance sheet assets. There's a lot of reasons - but the point is that the bank's, clients, largely, will influence the strategies/outlook/opportunities to manage assets. Most are more of a mix - they have a ton of stuff they do for bank clients or to help foster bank relationships, then they have other sets of products/services/strategies that they sell more stand alone (i.e. OCIO businesses or specialized client bases, like Insurance or similar). 

Aside from that - the other biggest difference is the bureaucracy. Banks are huge and they care about risk more than you, your unit or your business plan goals. They also share in the pain of bad years - efficiency ratio too high then your unit will share in the pain of cuts even in a bad year. The Asset Management units we are talking about here, not the wealth management generally w/ retail, are generally barely double digit line items at best for banks revenue wise - much less profit wise. This is all more than you really care about - but the point is AM is not their primary business. Others that you mentioned are built just for Asset Management - they can make decisions that AM arms at bank's often can't. 

Career wise? Starting out - get the role wherever you can. I tend to think bank's offer a good starting off point as you'll have lots of access, a big name on your resume, and the day to day will be pretty similar. Spend a few years there, build your network, and then jump to a TRowe or Blackrock if that's what interests you. Your biggest decisions, IMO, are what exposure you want strategy wise, what your coverage areas are, what is going to give you the most hands on investment experience, best network, etc. - those are needle movers throughout your career.