Better COLDER than HOTTER!?

48F is much better than 75F

48F is not that far off from an ideal temp.

 

You would probably be safer with the fridge.

 

I'd put it in the fridge.

 

It depends on long you plan to cellar your bottles and which type of brew you're speaking of. If this really is for cellaring (not keeping for later), keep them in your closet (put your IPAs in the fridge, not those for cellaring).
Heat doesn't hurt beer, it makes it age quicker. Cold can destroy your beer. 48°F is definitely too cold for cellaring beer, even 55°F is actually a bit too cold in my opinion. 60°F seems ideal for most styles.
75°F is pretty hot, I would suggest packing your beers in something like a closed cardboard box inside your closet to protect them from daily temperature variations. As they are in your closet they are safe from light, so then you can keep them for years.

 

long term aging (2+ years)? fridge.

short term? probably doesn't make a difference / closet better, though YMMV.

the thing is that heat speeds up "bad reactions" more than it speeds up "good ones" (supposedly). so, better to be colder than hotter for the long term. if it's not enough time for the "bad" reactions to happen, it doesn't matter.

p.s. pulled the 2 year cut off right out of thin air, if anyone's wondering.

 

If you want to preserve you beer in its current state, if it is filtered, and/or if it is a style not meant for age- put it in a fridge. If your beer has active yeast, and you want to see it mature- then keep it in the current closet so it can experience the temperature fluctuations as is natural. 75 is not too hot, 48 is good for putting the yeast into stasis and inhibiting bacterial growth on food.

As a brewer, and having a rather large "cellar" in the garage, I can say your current closet temps will not harm your beer. Try to keep the summer temps below 75 as much as possible, but if you keep your ales at fridge temps you will put the ale yeast used in those ales into hibernation. They don't operate that cold, they will hybernate, and you will not get the benefit of having yeast in the bottle.