C.P.E. Bach's Essay on the True Art of Playing Keyboard Instruments. Unfortunately, there does not seem to be a public domain English translation (you might be able to find one in a university library), so I checked the first edition German version on IMSLP (which I can't actually read). As far as I can tell from a quick skim of it, he appears to use the slashed-6 quite frequently. Now, you might argue that this is not modern typography -- which is true. But consider that C.P.E. Bach's work is (1) regarded as essentially the definitive treatise on continuo realization by modern scholars, as well as (2) the fact that he was writing at the very end of the Baroque (it was first published in 1753), so continuo styles had pretty much evolved as far as they were going to. In light of this, I think that following his conventions would be at least a safe choice.
My modern Dover edition of Handel's Messiah. This used a 6♮ to raise what would otherwise be a flatted note (e.g. B♭ in the key of F). However, in sharp movements (e.g. in D), the slashed-6 was used to raise what would otherwise be natural.
My modern Dover edition of Bach's Brandenburg Concertos & Orchestral Suites, which only has figures on the 5th concerto (in D, for the non-solo sections) and the 1st two Orchestral Suites (in C and D). These consistently use a slashed-6 to indicate sharped sixths. However, I did find one instance of a 6♮ in the 1st Orchestral Suite. This was an interesting case, because it was actually a cautionary accidental. The key signature was C, but the previous measure had a B♭ in the bass part, so the measure in question had a D with a cautionary natural next to the six. At any rate, this matches the use of 6♮ in Handel to raise a flatted note.
In no case did I ever see either a 6+ or a 6♯, so from this limited sample (two modern Dover scores), it appears slashed-6 is the preferred way to go... at least if you have a way to support the symbol in your software.