SRAM groupset equal to Ultegra R8000 series?
This is an easy question to answer: SRAM Force is equivalent to Shimano Ultegra. Here is one link from BikeRadar. Some sample quotes:


  Going up in price to Ultegra-level, SRAM’s Force group uses lightweight materials such as high-grade alloys and carbon fibre to be a very competitive gear setup. ... In 2019, SRAM added Force eTap AXS to its range, a 12-speed wireless groupset which competes directly with Shimano Ultegra Di2.


How about this variation on the original question: does Force provide a similar value as Ultegra when you compare it to the top-tier group? Probably yes. Bike Radar says that the eTap (i.e. electronic wireless) groups share the same motors, but Force has cheaper materials in some parts, and some items are constructed differently. Notably, Red's power meter is integrated with the chainrings, so you buy (and replace!) chainrings plus power meter when any one of the rings wears out. This is a new development and SRAM says it improves accuracy. At the same link, Force has the traditional setup where the power meter spider is separate from the chainrings (i.e. similar to the spider-based power systems from Quarq and Power2Max, which have good reputations). Force also has more limited chainring options that are more appropriate for amateurs (whereas Red's larger chainrings cater to pro cyclists). Functionally, the testers in the links above report that Force seemed very similar to Red. I believe reviewers would say the same about Ultegra vs Dura Ace, and about Force mechanical vs Red mechanical.

The answer is a bit more complex if you consider Campagnolo. If you just picked the second groupset from the top, you'd compare Ultegra and Force to Record. This would be misleading. Those groupsets are comparable to Campagnolo Potenza, as evidenced by this review on Cyclingtips comparing Potenza to Ultegra 6800 (the then-current Ultegra version) and SRAM Force.

I will find a source for this if people ask for it, but my impression is that Chorus, Record, and Super Record are functionally the same, but the materials get more expensive as you go up. Super Record has a bunch of titanium fasteners, a titanium crank spindle, and multiple ceramic bearings. Record loses some of that, especially the ceramic bearings. Chorus arguably compares with Dura Ace and Red. My impression is that Record and Super Record aren't functionally better than Chorus, but they are a bit lighter, and they're much more expensive. In contrast, Potenza has a different shifting mechanism than Chorus (Potenza and Centaur have Powershift, which lets you go down one cog per shift, but Chorus and higher have Ultrashift, which can dump several cogs per shift). Potenza and lower groups also use aluminum in some derailleur components, whereas Chorus has some carbon parts in the rear derailleur, and Record and SR get more carbon. Potenza also doesn't have an electronic shifting version.

Does Potenza provide a better value than Ultegra? The review I linked said that both groups were about equal, and that's consistent with other reviews I've read. Potenza will likely provide a similar relative value (when compared to the top tier Campagnolo groups) as will Shimano Ultegra or SRAM Force. I believe that in the US, Potenza retails for more than Ultegra does, but note that not many riders buy groupsets separate from frames and install them. So, for an average rider with no strong preference about shifting mechanisms, Potenza probably isn't as good an absolute value as either Ultegra or Force. This probably generalizes across other groupsets. For example, Bianchi's Aria road bike with a Centaur rim brake groupset has an MSRP of US$3,000. The 105 rim brake version is $2,600. The 105 disc version is $2,900. I believe all the finishing kits are the same across these bikes; the wheelset for the 105 disc version is different than the Centaur and the 105 rim versions (the latter two share the same wheelset).

There are soft factors that can sway a decision one way or another. Potenza mechanical shifters don't have adjustable reach, which pushed me away from Campagnolo groupsets in general (although Campagnolo hydraulic brake levers  have adjustable reach). Some people do genuinely prefer Campagnolo's shifting mechanisms to Shimano's. Some may prefer the shape of one brand's levers over others. It's been my experience that Campagnolo's rim brakes modulate better than Shimano's (i.e. with Campagnolo you can vary your braking power more finely, whereas Shimano's brakes tend to develop power quicker and tend to be a bit more off/on as a result; this is based on my opinion from having operated both Campy and Shimano groups and from statements of others). Potenza and Centaur are available with a matte silver finish, which can go better visually with steel or titanium bikes. The highest level Shimano group with a silver finish is 105.