It really depends on what is important to you. If you want a lot of customization options,
Textra is the way to go. I think I paid $3.50 ($6.50 now) for the Pro (lifetime ad-free) version of Textra back in 2017, so, I have definitely gotten good value for my money. If you don't pay for Pro, you will get ads. Because I have always used Pro, I'm not sure what getting ads really means for the user.
On the other hand, Google Messages is free, has RCS and a spam filter, while offering little in the way of customization. Although I have used Google Messages, I can't speak to how effective the spam filter is, since I only get about 10 spam messages in a year. Some here have reported that the spam filter works really well for them.
Check out the link I have provided for Textra to determine for yourself if it has features that would appeal to you. Or, download if from the Play Store for a free 14 day trial of Pro (ad-free).
For me personally, Google is going to have to do a whole lot more with Google Messages to make it an attractive alternative to Textra.