Both have pros and cons. Indices avoid risk from company-specific news (though sometimes this type of news can be good for your position, long or short): the company will move much more than the index. Large cap companies and indices are both very liquid, so have reasonably tight spreads and looser magin requirments. Indices are tradable outside underlying market hours. Some indices, such as the FTSE100, are more volatile, more reactive to US markets etc. and often reverse intra-day: much London price action takes place before the open, leading to a gap, which then steadily fills all morning until the US markets start quoting pre- their open, throwing the whole game up in the air again. Many London large caps are also very sensitive intra-day to oil, the Dow and the $.
Suggest you find on market that is appropriate for your approach, paper trade it, then micro-trade it. Once consistently winning and you have an adaptable system, expand position size, and only then try to identify a second and third market / sector.