I can find you people who will tell you that anything less that identical batteries, down to being the same age, is substandard, and technically they are right. Any differences in the characteristics of the batteries makes things less than optimum. However, not much in life is ever truly optimum, and you can decide if small and hard-to-calculate compromises are worth spending extra money.
Remember that when you connect up the two batteries, they'll equalize their voltages. Thus, when one battery, for whatever reason, charges or discharges differently than the other, there'll be voltage flowing around trying to get things equalized in a way that's probably not optimum for either battery. Does it make any real difference? Probably not in an under-the-hood application. The key point is that AGMs and flooded cells have nearly the same charging characteristics--definitely NOT the case for gel batteries, BTW--and thus they can replace each other. However, every battery has one perfect charging program, and with mixed batteries you won't be hitting it. But in charging off the alternator, you weren't at that level of precision anyway. (As an example, on the Odyssey AGM house batteries in my Sprinter that are charged by my Outback Power inverter, there were about ten individual parameters that had to be set to two decimal places to optimize the charging for those particular batteries.)
One thing to consider is whether the batteries are likely to end up being completely discharged. AGMs will bounce back from many total discharges, but two or three total discharges (and sometimes just one) will kill a flooded battery. If there's a chance that something might happen (lights left on, significant phantom draws, etc.) to totally discharge the flooded battery, it might actually end up cheaper to have an AGM that can recover.
So, as you might suspect, the best would be to invest in two shiny new Platinum Diehards and put your blue top into backup service. But if you're not doing that, it might be best to save money on the current setup so you can have the matched set ASAP. Ideally, you don't want to go through "one's new/one's old" cycles forever. (And whether to put a flooded cell into the mix would have a lot to do with how likely it was to be ruined by a total discharge.)
