what is the difference between a proper L3 interface on an L3 switch & a switched virtual Lan interface on L3?
which situations would justify either of them.
Yes, having the two links terminate on different modules would avoid failure of one module dropping both links.
If you go with SVIs (assuming you have spare ports), you would have two SVIs, each defined with two ports, one SVI port (of each SVI) defined on each module. Each external link to it's own SVI but initially not on the same module.
SVI A - ports mod1/1 mod2/1
SVI B - ports mod1/2 mod2/2
Link X connects to mod1/1
Link Y connects to mod2/2
Routing shouldn't be impacted.