DS is full of quality of life improvements. StB is very rough, a product of earlier era and it was somewhat experimental. DS had an advantage of knowing the grounds better and didn't squander it in this regard, except the replays: it lists 5 more slots than in StB, but there is a bug preventing you from accessing the last five; you can save, but not watch. In StB, if you don't clear the scene, you can't view photos that were taken beyond the miniatures. You can't save any of them as the decisive photo. Only cleared scenes allow you to view photos and change the current decisive photo by pressing s. You aren't given the exact score calculations when you watch through the photos after completed scene, just the score for that photo. All of that is different in DS, you can look at your photos no matter what, you can save them even if you didn't clear the scene, you are told how the score for that photo was calculated. For decisive photo you can later view the score calculations by pressing s. But the most important difference is that you are actually given your base score for any photo, instead of being stuck with calculating it yourself and guessing what exactly happens to any fractions at each point of calculation. This is good, it helps with understanding what is valuable from point perspective in a given scene and what is not. Both games also display some parts of calculations immediately after photo. In StB you are given only flat bonuses and modifiers, in DS there is also base score and final score. Furthermore, DS always has at least a flat bonus for direction and angle modifier and can have even more possible lines, which brings me to the next point.
DS is visually cluttered. After somewhat retro feel that you can get in StB going to DS felt as if looking at visual vomit, even from the very beginning. For final spellcards you will probably adjust and the patterns itself are complicated enough in both games that the difference is not that striking, but it is still there. The aforementioned score calculation text in DS is almost always longer, taking up space. Whenever you take a photo, in the middle of a screen you will be presented with "REST X", where X is the number of photos left until scene's completion. I have more than once failed a scene because that text hid a pellet from my view and it hit me. An absolutely unnecessary addition, because I've never had a problem with knowing how many photos were left in StB. Between just knowing the scene you are playing and quick glances in the moment of taking photos it was more than enough. Both games include a shutter chance, a moment in which you will get nice photo modifier to the score if you take a photo in that moment's time window. There is an audio cue, shared in both games, and a visual cue. In StB, it is a red magical circle that appears centered on boss, then quickly shrinks. It's very visible even with scenes that have a lot of danmaku near the boss and highlights the right moment exactly right. In DS, the visual clue is a bunch of white, interwoven polygons that due to lack of thickness can be easily missed out when there is a lot of danmaku around. Catching shutter chance is, like many things in Touhou, a mixture of remembering the spellcard and ongoing use of reflexes, in DS I found myself more than once in situation in which I could only rely on the former. A red magical circle from StB is still there, in fact it is always there, for some reason highlighting constantly position of the boss. Another unnecessary addition, because it was never a problem in StB to locate bosses, not to mention that your viewfinder directs itself on them without your imput with just pressing shift or waiting a bit. It didn't cause me to fail a scene, so it is less problematic, but it still can distract, because it also spins constantly and distorts the background for some reason. Many fancy bullets, perhaps too fancy at times, likewise more pronounced animations. Finally, whenver you start a scene in DS, you are greeted with big banner with "SHOOT!" written on it, with another banner containing name of the boss coming shortly after. Boss won't start attacking during that part and you won't be able to charge up into shooting during that time, but you can already move and charge. In comparison, StB simply unfades you from black as boss enters the playfield, with little fanfare, giving you the exact same freedom to act. In pure mechanical gameplay terms it is therefore the same in both games, but I'm pretty sure that this added stimuli in DS tires your mind just a little bit faster.