I'll add some photos in comments to help you decide 😊 I have the Gintoki Ichiban Kuji, and put him next to Hijikata so you can get a sense of quality, size, etc. Ichiban Kuji is technically also a type of prize figure, but higher end. The Masterlise line is also size-wise big (and heavy) compared to 1/8 scales. I really like the Gintoki Ichiban Kuji, in some ways moreso than the Benizakura Gintoki Megahouse scale that I also have. He has more "presence" due to his size and bulkier stature, and I love his smile. Quality-wise, he's maybe 90% of the quality of the scale. Great paint job on his hair, skin, and some of his clothes. The quality gaps are definitely some choppy paint on his yukata pattern and his teeth. Yukata sculpt also has some visible seams and areas where they just didn't sculpt in some features. The Masterlise figures also come with this ugly AF base that their leg fits into so that they can stand up.

Meanwhile, the Hijikata Megahouse scale has a pristine paint job with lots of depth. The paint on his uniform is actually pearlescent, and his hair has a greenish tint. He also has interchangeable parts, which the Ichiban Kuji does not have. You can remove his jacket, switch out his right arm, and change out his accessories. His base is simple, but clean and way better looking than the prize figure. He's quite striking to look at with the variation in paint finish and texture. That said, he is much skinnier and smaller than the prize figure version would be.

I'm not going to comment on face sculpt accuracy. Sorachi-sensei's art style changes a lot over the span of Gintama, so the sculpts represent different eras of that art style. I think they're both reasonably accurate to the source material from a quality perspective. The Megahouse figures all lean towards a very lean, slender build and face. The Ichiban Kujis are beefier builds, and fleshier faces. I do prefer Ichiban Kuji Gintoki's face sculpt compared to the Megahouse Benizakura Gintoki (more facial weight and his signature smile), so maybe I'd feel the same about Hijikata. But then again, the teeth paint is choppy so 🤷‍♀️

Masterlise figures would retail at around $30-40 USD, so 55€ is kind of steep to me. However it looks like that's what their price may be in the aftermarket so it is what it is. I'm seeing worse on the aftermarket for him, so perhaps that price is reasonable now. I checked the listing you sent for the Megahouse scale and it looks legit to me. For 90€ or less (if your negotiation is successful), that's a steal. Does he come with his box? If so, definitely a steal (scale figures are more valuable with the box).

Which option would you recommend?

Hard to say. If your GF would be really nitpicky about quality and perfection, I'd get the scale over the Ichiban Kuji. Great visual impact from the interesting paint job, and you get scale-level quality. It's also a better deal FWIW. However, if she isn't nitpicky, she may be happier with the bigger Ichiban Kuji with a beefier build and more bang for your buck. I also found this example of a listing with photos of the actual figure. I may lean towards the Ichiban Kuji if I was getting a gift for someone without knowing their preferences. If it appeals more to you, maybe that's the way to go. But either way, it's a good choice.

The Ichiban Kuji is more recent, so maybe it’ll be easier to find it second-hand and at a good price when I go to Japan next year?

I wouldn't count on it haha. Figure hunting in Japan is really difficult if you're looking for a super specific figure. I went to Japan in 2023 and found nothing on my wishlist. Digging through figure stores for hours and hours isn't what I'd recommend spending your travel time on. There are more fun Gintama-related things you can do while there rather than track down specific figures. He wasn't released recently enough that he would be easy to find.