The 6 or 8 pays $7 on a $6 bet. Paying true odds, a $6 or 8 should pay $6 on a $5 bet.

If the bet was $90 on a 6 or 8, the casino would pay $105. However if the casino paid true odds, a $90 would pay $108. The house is holding almost 3%

Now take the case of the 4 or 10. If you 'buy' the 4 for $25. The 'buy' usually cost $0 in Vegas. The casino payoff is $49. The true odds payoff is $50. The house is holding only 2%

Therefore, the 4 or 10 is the best place bet if the buy is free.