DirtyDovy, I don't know much about slug from personal experience — I haven't gotten much actual slug shooting in yet....
I do know about what slugs are currently available to US shooters.
Observations (1) Almost all of the 210+ slug models/weights offered by 11 manufacturers are available in
.217 and
.218 diameters. And some make exclusively
.217.
(2) Hardly anybody is making smaller sizes, like
.216.
(3) Rare exceptions are the following
16 slug models:
(a) JSB (1x | 25gr)
(c) Any day now: NSA (5x | 5c a shot)
(d) Altaros (1x | 31gr heavy)
(e) Mr. Hollowpoint (9x | 50c a shot)
Conclusion (1) It seems that manufacturers have determined in testing that the largest percentage of guns they are "targeting" for slug use like
.217.
(2) You own two FX guns, clearly you are part of the target group of the slug makers.
(3) Since JSB gives you the choice of .216 and .217 only, no .218, I would bet my buy-4-get-1-free order with all
.217
(And if you like to play it super safe, get one of the four in .216 — in your case I see no need, you've got a good hand.)
Matthias
PS: In my own slug shooting I have found both .217 and .218
not doing so great in my LW 17.5 barrel — so I will bet my money on the .216 for my order later today.