What's better for a rear main seal leak -- Bar's or Blue Devil?

Neither! 

Just fix the rear seal! 

Check your PCV valve first. If it's plugged or stuck, there will be pressure in the crankcase instead of vacuum, and that often makes the rear seal leak. 

If you have a worn engine, then blowby pressure past the piston rings will pressurize the crankcase too, even if the PCV valve works.  There may be no stopping a rear seal leak in that case. 

-Engine overhaul mechanic and general automotive mechanic since 1972

Rear main is the most misunderstood seal. I bet it could be oil pan instead.

Sodium silicate

There is no stop leak product that works on rubber seals. You need to replace the main seal and inspect the crankshaft for grooves. Make certain that it's not a bad valve cover gasket leaking down the motor os a bad oil pressure warning light sending unit leaking first.

A new seal is better than any of those products . 

Fix it properly.  Replace the rear main seal.

Both Blue Devil & Bar's Leaks both work in a similar fashion, in that they 'swell' the rubber in seals to reduce or stop leaking.

Depending on how bad the leak is, either product can have at least a marginal affect on a leak for a while.

2-3 ounces of DOT 3 brake fluid added to your engine oil will do almost the same thing.

To add; the rubber used in automotive hydraulic brake systems is a unique compound that is not affected by brake fluid, unlike the rubber used in engine seals, which will swell up when in contact with brake fluid.

Of course, replacing the bad seal is the best & longest lasting repair.

I honestly don't think it matters. Rear seals now are different then back in the day when they were a kind of rope that needed to be kept oiled. So it is a modern car I suspect it is just a waste of money.