connecting 2 routers

> [...] I want to connect the C7100V to my existing R6300V2 modem. This
> will provide me with additional ethernet ports which I need.

   Your "my existing R6300V2 modem" is a router, not a modem.

   Did you intend to use the C7100V as a modem or modem+router?  If
modem+router, then a cheap gigabit Ethernet switch could be used to add
more Ethernet ports.  In such a situation, configuring the R6300V2 as a
wireless access point would avoid the problems you might get by
cascading multiple routers.

> [...] Since the Netgear C7100V & R6300V2 they will be sited next to
> each other on my desk I could connect them with an ethernet cable or use
> bridge mode as explained in the R6300V2 manual.

   A cable connection should be simpler, generally faster, and less
subject to radio problems (interference, ...).  Why tie up the C7100V
radio(s) to talk to a R6300V2 which is close enough for a cable
connection?

> Question: Is the bridge mode more secure? Does it provide faster
> thruput speed?

   Radio more secure than a cable?  I don't see how.  Faster?  Unlikely. 
Less reliable?  Sure.

   But, if the R6300V2 will be very close to the C7100V, then it might
make the most sense to disable its wireless stuff completely, and use it
as a simple Ethernet switch.  That is, configure it as a WAP, but
disable its radios.  (Or, get a cheap Ethernet switch, and put the
R6300V2 away until you actually need its wireless capability for
something.)