Both are really good. However, SBR has limited returns with a built in risk of being shot down. One thing that some of the odds calculations I’ve seen here don’t take into account is the upper limit on damage for bases and MiCs which would limit the value of rolling your 5s and 6s since the surplus damage would be lost (max limit 6). When you take this into account, and the fact that a SBRing bomber cant strike elsewhere, and also that you may well capture the MiC (esp Bombay or Leningrad) already damaged and have to repair it as the captor, and the whole thing can be seen as having limited utility with one caveat. If the SBR is used to consistently overwhelm one enemy (Russia), all at once and turn after turn, it is devastating because they cannot recover. Then it truly is “strategic” because it leads to their national defeat rather than penny packets of damage.
The convoy thing is obviously awesome, but it relies on having some form of naval+air+sub supremacy already. This is probably the weakness to the sub strat that YG and we were discussing last week; you cant really even get to the convoy spaces, much less sit on them, unless you have already driven his fleets away or destroyed them. so, its not really something you can apply as a “strategy” because your primary strategy has to be control of the seas (rather than dividing your forces up to damage his convoys.
The key to both is using them in combination to overwhelm the enemy, not losing 1 bomber or 1 sub at a time or doing 2-3 of damage incrementally.