We strongly discourage "X vs Y" threads here, but I'm inclined to let this one stand.. It's part of a specific query, seems to be a genuine attempt to seek information, & I think it could lead to an interesting discussion.
I can't say which is "better". The L-39 sold far more, but part of the reason for that must be that it was adopted as a standard trainer by the Warsaw Pact (following on from the L-29), so was built in very large numbers just for them. It was also included in Soviet military aid packages.
The S.211 was a private venture, and came into the market later. Without a domestic order, it faced a much harder task selling. It can't have been bad: Singapore bought it, & the RSAF is famously choosy.
They're aimed at rather different market niches, I think. The S.211 is much smaller, & the niche it was built for is now generally filled by turboprops. I think this contributed to its small sales. It came along at the wrong time, when some good turboprop trainers came onto the market, & were very hard for it to compete with. Singapore replaced it with turboprops.
The L-39 was aimed more at the niche now occupied by aircraft such as the Hawk & M346, training to fly fighters.
I agree with EXSSBN2005 on the second question.
