Forum Moderators: not2easy
As you said, THEY PROBABLY ALL WORK. The important thing as everyone offers you advice is to remember that. Otherwise you'll get more, not less, confused. :)
DIV FLOAT PROS:
-Div styles are pretty reliable cross-browser.
-You say "images with captions underneath to start with". If you're planning on adding more complicated stuff, having each gallery item inside of a div may be helpful.
-For those who don't know a ton about how CSS behaves, it may be easier to set <div>s' styles than <li>s' styles. For example, when you set a <li>'s width to 100px, does that include the bullet point and the space between it, or not? Do you know the answer?
UL pros:
You can easily set a style for the whole gallery: that is, if you wanted all the captions to be font-size:11px, you could put that in the UL's style instead of each DIV's style.
DL: I'm not familiar enough with DL to say.
Keep in mind, you could even have <div>s inside floated <li>s inside a <ul>. (That's what I would do. That way, you can have the advantages of both divs and ULs, and an extra advantage: margins and padding will be easier to manage*.)
*If you didn't already know, css width sets the width of the content, and margins/padding are added on after that. E.g. given
#gallery {height:500px; border:1px black solid;}
#gallery li {height:100%; padding:5px; border:1px black solid;}
the <li>s would be set to 500px tall, and THEN padding would be added within their borders, so they would be 510px tall total.
However, given
#gallery {height:500px; border:1px black solid;}
#gallery li {float:left; height:100%; border:1px black solid;}
#gallery li div {margin:5px;}
and
<ul id='gallery'><li><div>content1</div></li>
<li><div>content2</div></li>
</ul>
the <li>s would remain 500px tall and there would still be a 5px space between "content(1,2)" and the li's border. This may or may not be useful; depends on your individual gallery.
So: I'd recommend divs inside floated li's; but then I don't know just what you envision your gallery looking like.
Otherwise, between floated divs and UL's: if you put all the divs within a container div, the container div could be made to act like a <ul>; meanwhile, <li>s are containers just like any other and can be made to act like <div>s. So what it really comes down to is your personal preference.