Given the two voltage choices, I assume that the battery packs you are looking at are Nickel-Cadmium. They are made up of one or more individual cells. The nominal voltage for a healthy Nicad cell is 1.2 volts DC. Hook four of these cells up in series and you will get a 4.8 volt battery pack (1.2 V x 4 = 4.8 V). Hook five of these cells in up in series, you get 6.0 volts (1.2 x 5 = 6.0).
A 4.8 V pack has four individual cells in it, the 6.0 V pack has 6.0 volts in it.
Most RC systems are sold with the 4-cell 4.8 Volt battery as the standard receiver battery. People have been using the 4-cell pack for decades in RC equipment with good results. The usage of the extra cell (6 volt pack) came about for a couple of reasons. First and foremost, the extra voltage will tend to increase the available torque of the servos. Secondly, the additional volt or so will tend to decrease the end-to-end transit time of a servo (speed up the servo).
A bit of security is also involved with the 5-cell pack. If you're flying and one of the four cells in a standard 4.8 volt pack dies (goes short or "0" volts) you've suddenly got a big problem. Your receiver usually isn't too happy with a three volt source and things just quit working -- and you have a dandy but spectacular crash on your hands (it happened to me more than once with three fairly new JR 1000 mah battery packs in the middle 1990's).
The downside of using a 5-cell pack is that you most likely won't notice that you've got a shorted cell in a 5-cell pack unless you're looking for it with a voltage monitor, or you just happen to notice that things are moving a little slower. With a 5-cell (6-volt) pack, you run the risk of having what is called an undetectable latent failure. In other words, you may not notice that the first cell has gone bad until a second cell in the pack fails, and then it may be too late. With care and a good battery maintenance program, this is not a big problem.
Looking at the manufacturer's literature, they typically consider a cell discharged when its voltage drops to about one volt from that nominal 1.2 volts. This means a 4-cell pack is discharged when it reaches 4 volts. A 5-cell (6 volt pack is considered dishcarged when it reads 5 volts. You may not get much extra flying time using the 5-cell pack over the 4-cell pack, since when a Nicad reaches its "discharged" state, its voltage drops off rapidly to 0 volts.
What is MAH???
It is an abbreviation for "milliamp hour" -- the measure of how much energy the battery can store and deliver.
An ampere is the amount of current that flows in a circuit when a one-volt source is connected to a one ohm load. (Current [Amperes] = Voltage [Volts] divided by Resistance [ohms].
A millamp is 1/1000 or 0.001 ampere. (1000 milliamp = 1 amp).
A battery pack rated at 500 mah is capable of delivering at least 500 millamps for one hour. It can also deliver at least 250 milliamps for 2 hours. Or it can deliver at least 100 milliamps for 5 hours -- by now I hope you get the picture. Again, the battery is considered discharged when its voltage reaches about one volt per cell.
Note that the "mah" rating of a pack is a "minimum" number for that pack when healthy. In most instances, a good pack will deliver more usable energy than its "mah" rating indicates -- that is, it will run longer before being depleted. When its capacity drops below that minimum value, you've most likely got one or more bad cells in the pack, and it's time to replace the thing.
So -- your 1800 mah/4.8 volt pack will allow a longer flying time than the 1400 mah/4.8 volt pack, since it has the ability to deliver an additional 400 mah of energy to the same load.
The six volt pack will provide you with a bit faster servo speeds, and with more holding torque than the 4.8 volt pack. If the 4.8 volt pack and the 6.0 volt pack have the same "mah" rating, they will provide the same amount of flying time, but the 4 cell pack will have an end of discharge voltage of 4 volts, while the 5 cell pack will have an end of discharge voltage of 5 volts. You won't necessarily get any additional flight time out of that extra volt simply because when Nicads are depleted, the voltage drops almost immediately to 0 under any load.
Also -- if you plan to use the 6.0 Volt pack, pay close attention to the radio manufacturer's warnings. I notice that Futaba warns that certain of its servo models are not recommended to be used with a 6 volt pack (you'll smoke 'em).
The bigger the "mah" number, the longer useable time you get from the pack. You'll get to fly longer on a single charge.
The bigger voltage will give you more torque and servo speed (but beware of the maximum operating voltage of the stuff you plug in, or you may cook something).
Battery packs of different voltages (6.0 vs . 4.8) but of the same "mah" rating will basically provide you with the same amount of safe flying time.
Oh, and finally, if you are using the stock charger that came with your radio -- you'll have to charge an 1800 mah/4.8 volt pack longer than a 1400 mah/4.8 volt pack simply because you've got to stuff an additional 400 mah worth of charge into it.
Keep in mind that the stock battery charger in your radio was probably designed for charging something up to about a 1000 mah/4.8 volt pack. (That extra 400 or 800 mah may just take too much additional time for that little wall plug-in charger). You may want to purchase a third party (aftermarket) battery charger.
If you're planning on using the stock charger designed for a 4.8 volt battery pack, it may not be capable of charging the 6.0 volt pack. If it is, it may take much much longer to charge the pack than you're willing to wait. You may be better off buying a third party (aftermarket) charger for the 6.0 volt pack.