A difference of one string, yet the 16x19 seems significantly more popular than the 16x18. Is there a significant difference between these rackets performance-wise?
Rackets with more vertical and horizontal strings – e.g. 18 x 20 – are described as having a “dense or closed” pattern, while rackets with less vertical and horizontal strings – e.g. 16 x 18, 16 x 19 – are considered to have an “open” pattern. All these string patterns are most common. As per your question, 16 x 18 and 16 x 19 can be considered as open pattern one and has a difference of one string only.
How open pattern affect on-court performance:Source
Power: Fewer strings result in more power
Spin: Fewer strings create larger “squares,” grip the ball more, and result in more spin
Feel and Comfort: Open patterns absorb shock better and are more comfortable
String Tension: Lower string tension will result in more power for all string patterns
An article from Tennis Warehouse University has done a research on Spin and String Pattern few important parts summary related with this question are:
Even if the two rackets are of same pattern in our case open pattern, tension also plays part in producing spin. Following two figures obtained from the website shows the spin ratio for open pattern with high and low string tension.(For pattern 16 x 18)
The pictures are obtained from the web and copyright belong to the author of the content and posted here for educational purpose only.
Summarizing, open pattern generates more spin than the closed pattern but even for the same pattern spin produce will differ because of tension of strings. Considering both 16 x 18 and 16 x 19 have same tension there will be little variation on produce spin.
At last there will not be significant difference between these two rackets performance wise considering both have same tension. It will ultimately depend on choice of a player, which one he feels comfortable with. But I couldn't find something suggesting 16 x 19 being popular than 16 x 18 pattern racket.