They are two different companies. BCE (Bell Canada Enterprises), commonly known as Bell, it a national company. It provides cellphone services, landline telephone, internet, cable, and media (it owns a variety of TV channels). Bell Aliant (BA) is a regional company, focused in Atlantic Canada. It provides landline telephone, internet (fibre-op), and cable. Though it does not have a national presence, it has been doing a good job expanding its fibre-op internet and cable services to rural parts of Atlantic Canada (as well as some of the more populated areas). BCE pays dividends to BA, which helps fund BA's high dividend payout.
I would consider BCE to be more of a stable stock, due to its size and diversification. BA is great to hold for its dividend, but it is more interest-rate sensitive and may be more susceptible to a decrease in share price if there's a large market sell-off. However, all things considered - if a "threatening" fourth player enters the Canadian telecom market, I could see BA as a great candidate for acquisition.
** Disclaimer - I hold both BCE and BA.