After people realize what the main differences are between the two techniques then they ask,,"which is better for me?" I'll put the pros and cons side by side to help you make the most informed decision.
Foil Highlights vs Balayage Highlights
-More coverage -Less overall coverage
-Less dimensional -More dimensional
-Smaller/finer highlights -Thicker, more "ribbon like" highlights
-Can lift up to 6 levels* -Can lift up to 3 levels*
-Higher maintenance -Lower maintenance
Conclusion:
Foil highlights are better for clients looking for maximum lightness or a more solid look all over. If dimension is required it usually comes in the form of a lowlight. Foil highlights leave a harsher line of demarcation (grow out line) so maintenance will be more frequent.
Balayage highlights are better suited for clients looking for a more dimensional, natural, low maintenance look. The way the highlights are painted on and diffused over the hair strand allow a softer grow out and more time between touch ups. Balayage however, is NOT great for clients looking to go from very dark to very light hair in one session, or someone that wants a solid color all over. Feel free to ask specific questions in the comment space!
*This is my personal experience and taking into consideration the integrity of the hair.