Let's see if we can compare the possible topologies.
Schematically:
Internet -> ONT -> 2.5 Gb ethernet ->RB5009 (ether1) ->10G SFP-PLUS->CRS310 (SFP-PLUS1)
NAS->10G SFP-PLUS->CRS310 (SFP-PLUS2)
Clients connected to other ethernet ports of the RB5009 will have 1 Gb connectivity (7 ports available)
Clients connected to any port of the CRS310 will have up to 2.5 Gb connectivity
No real need for any VLAN (unless it is wanted for other reasons).
vs:
Internet -> ONT -> 2.5 Gb ethernet ->CRS310 (ether1)->10G SFP-PLUS->RB5009 (SFP-PLUS1)
NAS->10G SFP-PLUS->CRS310 (SFP-PLUS2)
Clients connected to RB5009 will have 8 ports, of which 1 2.5 Gb and 7 1 Gb
Clients connected to CRS310 will have 7 ports, all 2.5Gb
BUT you will likely need VLAN(s), AND in this case you should have an offbridge port (ideally on both devices) so you will loose one port on the RB5009 and one on the CRS310.
I cannot see any meaningful differences in speed/bandwidth/performance using the one or the other? 
In both topologies one might (I guess it depends on the ISP and on the way it provides the internet) get rid of the ONT and use a fiber SFP-PLUS module "directly".