Both are correct gramatically and syntactically. As in most things Spanish, this has important regional variations in meaning.
In general the only difference between these two forms, is the mode of address.
Contacte con nosotros Is generally (regional variance details to follow below) the formal mode of address
Contacta con nosotros Is the informal mode of address
Contáctanos Is More of a relaxed informal, or familiar mode of address
This is traditionally a very common string used when writing web pages, and like in any written medium, you should follow a consistent mode of address throughout the entire document or page.
Surely everyone will understand the meaning regardless of the mode of address you use, even if you mix them, but doing so will cast a poor or amateurish impression on your readers.
To decide which mode of address is more appropriate for your writing, consider your audience and purpose of the document or web page.
Financial Institutions, Professional associations, and in general, services that have a very low turnaround, will use the formal mode of address, and with good reason! You are entrusting them (say a bank) with your assets, so respectful, factual and unambiguous communication is a must, and that calls for formality.
Public utilities and government sites will use the informal mode to give the impression of a responsible, yet dynamic organization in communications with their customers
If your main audience are young adults and adolescents, or the topic is about their iterests, (videogames, pop music, sports) then the familiar mode of address may actually generate more trust amongst them, because you "speak their language"
Some Latin American countries in Central America, (Guatemala, El Salvador) and the Caribbean (Venezuela) have inverted the formal and informal modes of address, an example of this inversion could be
"Papa, yo a usted le amo con todo el corazón".
Is pretty weird in Mexico, yet correct usage in El Salvador, a father being one of your closest and familiar relatives.
"Alfredo, apenas te he conocido y no puedo decir que te amo".
would be awkward in Mexico where a couple just met and the formal mode of address is expected until trust and familiarity is established, but would be exactly right in Venezuela, where this mode is used to denote deference, distance or respect.