Hello there!
This isn't a question that we can answer for you definitively, but I think I can help you figure out the best method here.
(Before we start, I want to clear something up: if you use a # in your URLs, Google only indexes the page as it's loaded with the URL before the #. So, if you make the page title change for t-shirts/#/?=1&filter.style=vintage_, Google won't even see that.)
To restate your question, and make sure I'm understanding you correctly: you're migrating a smaller site into a larger site. Your options are:
- The standard option, redirect pages to the most similar page on the bigger site
- A (slightly sneaky) SEO option, where you redirect pages to pages that are similar enough for good user experience, but are pushing link equity to the pages that need it.
My answer is: go with option #1 unless you can prove that option #2 will have an impact on rankings for the right pages.
To determine if you can make an impact, map out which pages you'd like to redirect where. For simplicity sake, assume that all inbound links to oldsite.com/vintage-tshirts and oldsite.com/hipster-tshirts will transfer full equity to biggersite.com/tshirts. Add those links to the current count of inbound links to biggersite.com/tshirts. Then, go through the top keywords you want to rank for, and see if this new number of links will cause your rankings to increase. (I like to use the Mozbar and actually search for whatever keyword I'm targeting, and look at the number of inbound links and the total DA of competing sites.)
Be sure to look for potential increases in traffic if you send oldsite.com/vintage-tshirts to biggersite.com/vintage-tshirts too - typically, you get more conversions for pages that are already better targeted to the query.
There's definitely some wiggle room here. If you think that the new inbound links are significant but they won't actually get you up a ranking, go ahead and go with option #2. If you find that you don't have a whole lot of inbound links to oldsite.com/vintage-tshirts, though, this probably isn't worth you time.
Hope this helps!
Kristina