I'd suggest getting CoD first just because online, if you don't get in on the ground floor, it's kinda hard to catch up level wise. You can probably play Mass Effect anytime, but if you hold out on CoD, everyone else might be light years ahead of you.
That's what I'd do though, but you might need a little break from the shooters on the other hand and Mass Effect would be good in that regard if you know what I mean.
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