Specifically on the hotel side it looks like there are a bunch of Hilton properties, a fair number of Marriott properties, and 1 Hyatt.
Recently Hilton and Hyatt promotions have been more valuable. It might be worth switching off between the two which could give you top tier status in both. Depending on your willingness / flexibility, you can also split your days or weeks between different properties in the same change to accumulate "stays" more quickly and reach top tier status.
I will point out in counter to the Delta/SPG thing, if you get Marriott Platinum you will receive United Silver. Likewise if you are United Gold or above, you qualify for Marriott elite status.
Spend some time looking at the earning rates of Marriott and Hilton as well - Marriott penalizes more for their small market brands (e.g., Courtyard / Residence Inn which earn half of full service Marriott's) vs. Hilton (which provides the full 10 points/dollar at Hampton but only on room rate).
If you do go Hilton, consider picking up the $95 Annual Fee AMEX Hilton Reserve or Citi Hilton Reserve, both of which give you Gold status from the start (though Gold isn't as valuable if you will be staying in Hampton Inn).