i found there to be almost no difference in service at the value to moderates... values offer luggage assistance and will bring your bags to your room, but not necessarily right away, or they will loan you a cart to transport your own luggage... the only other difference was the lack of a sit-down restaurant at a value, but the food court selections were plentiful and excellent, and for our family, if we are doing sit-down we are either in a park or a deluxe (i.e. o'hana @ the poly)... pools at mods have slides whereas there are no slides at values...
one thing i liked about pop century was the "small" feel of it, even though i realize it is quite big... we saw the same cm's multiple times during our stay and they made it feel very homey, plus we ran into the same families at the pool, on the bus, in the food court and that was great for the kids to make vacation friends... (maybe because we spent alot of time at our smaller pool/the bowling pin in the 50s section?)
anyway, i did not see a big difference in values to mods, and personally, the little extras that mods supposedly offer, are not things we "need" (or even want)...
i hope that helps...