i have a large web application, with over 20000 users (about 100 people using at any time)

now im making a new version in PHP 5.1 (on FC3 linux, apache2, mysql 5 server)
of the web app

i use PEARB (and other pear libraries alongside my php5 classes) for database connection

the app works fine with MYSQL and MYSQLI

which should i use? which is faster?

for me performance is the target thats why im recoding the system into php5
im also using alot of object caching to reduce database load

oh and almost forgot;
current database size is 800mb
im reduced that to 100mb by changing schema and using caching