Use Two or One Instance of WordPress for Main & Blog Site?

Hi,

I have a main site and a blog site, as below:

Main Site:

URL: https://www.example.com
Total Posts: 930
Daily Visitors: 980
Monthly Visitors: 29,400

Blog Site:

URL: https://www.example.com/blogs/
Total Posts: 3710
Daily Visitors: 17100
Monthly Visitors: 515, 000

Currently I install two WordPress instances for main and blog site separately, using different themes.

Now I am thinking two solutions:
1. Merge the blog site to the main site and use one WordPress instance only.
2. Keep two instances, but use same theme for them so that they will have the same look & feel. Letting others to belive they are in the same site.

Below is what I can figure out for the pros and cons for merging:

The advantages of merging:

1. All posts data are put in the same database so they can be searched and processed together.

The disadvantages of merging:

1. After merging, the total records in the database will increase which may decrease the performance.
2. Google Analystics will have 500, 000 visitor/month for each site. If the visitors go over this limit, some data are not stored. So a merged site will have more chances of loosing some data.
3. All operations, including link check, build index, scan, will take more time since there are more posts.