The difference is the software image licensed. The -L signifies LAN Base and the -S signifies IP Base. You can read up on the features of both from here: http://www.cisco.com/c/en/us/products/collateral/switches/catalyst-3750-x-series-switches/qa_c67-578933.html
All Cisco devices are eligible for SmartNet coverage - but you have to pay depending on how long and what type of service you want. All Cisco switches also come with a limited lifetime warranty now but the replacement time is usually 2-3 weeks in my experience.
