Under my proposal, the level-10 character (+5 stat, +4 prof bonus, +max[d8 expertise or d10 Bardic Inspiration]) would get an average bonus of +6.6. They'd have an 83% chance of beating DC20, 58% chance of beating DC25 under my way; with your way (adding bonuses), it's 92% and 74%, respectively. (For comparison, with just the d10 bardic inspiration and no expertise, it would be 77% and 52%)
I prefer the numbers with my way, but YMMV.
I prefer the numbers with my way, but YMMV.
