Immigration at LAS should be a breeze if you are in WT+ as you should be one of the first off the plane as it's a Mid-J 747. Personally unless there's a significant cost saving in flying through DFW or LAX I would always take the direct flight. There's an immense sense of relief in getting off the plane, clearing immigration and customs, and going straight home / to your hotel and not having to endure a further three or four hours festering at the airport and then another hour in the air.
In terms of the WT+ cabin itself, there is no denying it is a little tired and a little dated but I would assert that it's still better than WT and as the LHR - LAS flight would be a daytime flight it's less of an issue. One caveat is that there is usually just one CC looking after the cabin so the left hand side is served first - so if you're in 16K then you're last to be served. If you have status then you may find that the CC will ask what main course you would prefer and ensure that it's available.
Flying back to the UK via LAX would mean an extra flight but you'd only need to clear security / TSA in Las Vegas and then you can take the connector between T4 and TBIT at LAX as well as enjoying the lounges there. The lounge at LAS is functional but nothing spectacular.