A boatload of capspace doesn't mean they any add great players with it. That's not tangible. It's hope, conjecture, and hypthetical. The third pick in this draft might net the Celtics a guard like Hield, Murray, or Dunn (which doesn't address their biggest needs) or a project international big in that Bender prospect, who will unlikely make an immediate impact. So you're still talking about Thomas and Bradley (I don't think Smart is very good), a rookie who may or may not be any good, and a bunch of role players.
In Washington, you have a multiple all star at PG, a pretty good and young SG, and two serviceable bigs in Gortat and Morris. They're not great, but I take those two over what Boston has in the frontcourt with guys like Olynyk, Sullinger, Johnson. By adding Durant, Washington has less holes to fill than Boston, and the holes they would still have would be really off the bench depth holes. Adding Durant to Boston, they would still need two major upgrades at PF and C and could stand to upgrade Bradley at SG.
And in the Eastern Conference, it's so wide open and year to year, things can change drastically. Look at what Atlanta did from 2014 to 2015 without any major changes to the core of their roster. From 38 wins to 60 wins in one year. The biggest thing might have been Horford being healthy for most of the year. That's a similar issue with Washington from last year to next season. Beal missed 27 games. Morris was a mid-season acquisition. They were 3 games out of the playoffs with Beal sidelined one-third of the season and with Morris only playing the last couple months on their team. Things can change quickly in the wide open East. Every thing else equal, Washington's roster appears more attractive to me, despite the success Boston had and how bad Washington's record was last season.