So, the odds the spins are better in silver -> Gold -> Plat
well... if you can beat all leagues with the same efficiency, then the odds of spinning a rare are plat>gold>silver>bronze. (since you need fewer and fewer consecutive wins in each league to spin)
Of course, the difficulty of the decks are bronze<silver<gold<plat.
so you must figure out your winning percentage vs. the frequency of rarespins vs. the time it takes to spin for each league and choose the best one.
for example, if you have a deck that wins 90% in gold, and it can win 85% in plat as well, then plat is better for you.
same for all the other leagues.
My personal experience is that if you have a deck that wins 90% in bronze (meaning that, in 100 games you'll win 90 and thus will have 90/6=16 spins) then you might be able to win 80% in silver with the deck (winning 80 in 100 will earn you 80/5=16 spins) then you should play silver.
Of course this is not this easy to determine, since you'll need consecutive wins, deck tweaking etc etc. But you get the idea.