Reminds me of something a particular poster on this forum once said about Donkeys and farts...
Peace,
Greg
That's how you fill your spare pony, er donkey.
i might as well chime in and say neither
+1
To the original poster, just get out there and dive. Dive within your level and within your comfort zone. Pay attention to your gauges and do some reading on Gas Management. As I recall, both [user]Lamont[/user] and [user]NWGratefulDiver[/user] have some great articles on that subject. Study them, apply what you learn and just get out there and dive and you'll probably find that your desire for a pony bottle decreases significantly. If, after doing that, you still feel the need to have one, you'll be in a better position to make an informed choice on what's right for you, based on your dive habits and consumption rates. Ask 10 divers, you're gonna get a dozen answers, most of which wont apply to your situation at all.
Dive safe!