In my opinion, if you had to choose between a spouse visa or Tier 2 visa (and both were feasible options), I would opt for the spouse visa. I would generally only recommend a Tier 2 visa over a spouse visa if there was a reason the spouse visa wasn't an immediate option.
Some points to think about if you're choosing between the two:
- A Tier 2 visa is tied to her job. If for whatever reason it doesn't work out (redundancy, job unhappiness, wanting a career change, etc), she would lose her ability to remain in the UK with that visa. With a spouse visa, she is free to pursue any job and change jobs as much as she wants to.
- Permanent residency through a Tier 2 visa could be a changing goal post. There's a minimum income she'll have to be earning after 5 years to qualify, and the requirements may change and she won't be able to apply for ILR through this route when the time comes. There have been members of this forum who spent 5 years in the UK with a work visa, and in their last year the ILR requirements changed and they couldn't qualify. They had to move back to the US.
- Switching. If at any point she switches to a spouse visa for any reason, the clock to ILR resets. Any time previously spent on a Tier 2 visa will not count, and she'll need to accumulate 5 years in the UK with a spouse visa to qualify for ILR. If you begin with a spouse visa, you wouldn't need to potentially reset the immigration clock if you had to switch from a Tier 2 to spouse visa later on.
If you are applying for a spouse visa, it doesn't matter at all if she has an offer letter or not - it isn't counted. The spouse visa is entirely based on your (UK sponsor) ability to meet the financial requirements (save for specific exceptions, like a pension, significant savings, sale of a house, etc).