I've thought about it, and compared Flickr and Gallery according to several different criteria. What follows is a long and tedious list that smacks more of accounting than of geekery, and is of interest only to those who are wrestling with similar dilemmas.
1. Data presentation
1.1. Ability to organize images hierarchically
At first, Gallery had an advantage of having hierarchical albums with as many levels of subalbums as you want, whereas in Flickr you could have only one level of "sets". But Flickr recently introduced so-called "collections", which can be made up of sub-collections and sets. Thus it now provides a hierarchical way to organize your images. So Gallery and Flickr are pretty much equal at that point.
The score so far: 1:1
1.2. Ability to present newest images first
Flickr has a significant advantage: it presents your pictures as a (photo)stream -- newest pictures first. (That does not keep you from grouping them into sets and collections.) On the other hand, the only way to access newest pictures first in Gallery (I suppose) is through an RSS feed. And I'm not even sure if Gallery supports RSS feeds! (Though I've read somewhere that it does.) But try to explain to my mother-in-law (who regularly checks my albums for new pictures of Erika) what an RSS feed is, or how to access it! (My own mother, OTOH, took to RSS feeds with gusto after I explained to her what they were.)
The score so far: Flickr 2, Gallery 1
2. Hosting: on my website vs third-party website
One can say that an advantage of Gallery is that it is hosted on my own website, where I have "full control" over my images and data, whereas Flickr is hosted by Yahoo, a big, impersonal, corporation -- and we all know that big corporations are just one step shy of becoming Satan incarnate, and are constantly thinking of ways to harness their customers' data for world domination. :-) Well, the reason I put "full control" in quotes is because I'm afraid the control I have over my data on my website is illusory. First, even though it's "my" website, it is hosted not by me but by a corporation (Dreamhost), and they are the ones who have the actual control over it. Second, performing an upgrade that hosed my software actually caused me to lose control over my data. Loss of control over data comes not just in the form of having your data turned over to FBI, but also in losing ability to manipulate it due to software glitches. Whereas if Flickr hosed its own software, I'm sure they would be working day and night to fix it.
The score so far: Flickr 3, Gallery 1
3. Community aspect
On Flickr I am a part of a big community that occasionally shows it's paying attention to my pictures by way of commenting on them or marking them as favorites. It's nice. The Gallery on my website is not a part of any community. Sure, Gallery has community features: it lets you create user groups, and it lets users add comments to pictures. But not surprisingly, no one has ever created a user account in my Gallery just so as to comment on my pictures. Every time a viewer needs to create an account to participate in something, the probability of their participating falls by about an order or two of magnitude. On Flicr, though, all the Flickr'ites already have accounts, and with those accounts they can participate in all the numerous communities.
The score so far: Flickr 4, Gallery 1
If the score is so much in favor of Flickr, why do I even bother thinking of fixing Gallery on my website?
Here we come to the critical feature that makes it very hard for me to give up Gallery. It's not a feature of Gallery per se as much as a corollary of the fact that I'm hosting Gallery on my web site vs third-party website.
4. Access logs
My web hosting provider allows me to see statistics of who accessed Gallery and when, what images they viewed and where they came from. I am a sucker for access logs. I want to know which IP addresses are viewing my images, and how the viewers found it (referrer pages and search terms). That's 50% of the fun of posting images. I want to know much more than just the view count on a particular image. Because of that, I am not ready to give up Gallery.