by the time you can afford one, i'm sure some spooty-uber will have bought them all and posted a detailed comparison somewhere ^-^.
but, back in the old days, the more expensive mounts did go faster. there were 4 levels, ranging from just barely faster than base run speed with no buffs, to something like 3x that :x. they ranged from 11k to um... 140k?
it's likely, yes, that some will be faster than others, and it's likely to relate to the price.
something else to consider: in eq1, your mount ate and drank from your food/drink inventory. in addition, there were things that could not be done while on a mount. just noting things to watch for :).
i havent examined any of the whistles or trinkets in that merchant's inventory to see if they say anything, so it's hard to say what they've done yet.
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EQ: Runyaessz, 5 Monk, Phinigel
EVE: Runym/Runyn/Runyl, 43M SP each
SWTOR: Pardoric, 22 Merc, on vaca
EQ: Runyariel Varyuvantel, 65 Enchanter, Tarew Marr, on vaca
LoTRO: Runyarian, 38 hunter, Vilya, on vaca
WoW: Runyarian, 70 hunter, Muradin, on sabbatical
EQ2: Runylala, 25 Troll Bard, Qey, Blackburrow, retired
WAR: Runyael, 30 DiscO Khaine, Ironfist,retired.
CoH: Hissing Chicken, 16 Scrapper, Pinnacle, retired