At least from what I've noticed:
There are more Ice planets than all others, but the planets tend to be pretty small. I loaded a Large map, and only counted 7 different types of Huge Ice planets.
Gas planets are obviously the biggest, none being smaller than Medium. However, there are fewer Gas planets than all others, and no moons are ever Gas planets.
Rock planets are an average - Average amount of them, average size.
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In Single-player SE4 (and I imagine in PBW aswell), Rock-planet Empires are the most common choice - so competition for planets can get rather fierce.
Because there are so few Gas planets, you could probably have just as fierce competition for them, if only half as many players started on Gas planets.
With Ice planets, not only are they the least favorite of the 3 (meaning more planets for you), but they're the most numerous. More planets also means greater chance a planet will have your atmosphere - with an Average Environmental Resistance, A Tiny planet with a breathable atmosphere is exactly as good as a Huge planet that's not breathable. Since there are more Ice planets (or so I think), there's a greater chance that one will be strategically placed within a Galaxy (i.e, right next to a Warp Point). Lastly, more planets means more Space Yards - No matter how advanced or fast your Space Yard is, the cheapest ships will never be built in faster than 0.1 years. But if you have, say, 6 Spaceyards in a system, you could build 6 ultra-cheap ships in that system in 0.1 years!
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And there's my completely unbiased comparison of the three planet types
[ May 08, 2003, 17:25: Message edited by: Dingocat85 ]