想请问拿工作签证可以申请美签吗？和学生身份申请有什么不一样呢？

只要你准备好大使馆需要的资料，就可以了。
没有工作签证和学生签证之分

是不是要单位出证明，对内容有什么要求吗？

[url]http://www.unbsj.ca/international/immigration/Whattobring.php[/url]

The following documents are generally useful to help establish ties for all applicants and must be presented when applying for a non-immigrant visa (submit originals only):

[SIZE="4"]A current letter of employment (or that of the family member or person providing you financial support) showing date hired, salary, full-time or part-time, and position. [/SIZE]
Pay slips and tax documents.
Current school letter with proof of tuition payment and transcript of marks for current and past year.
Property deeds.
Canadian permanent resident card.
[SIZE="4"]Work authorization or study authorization/permit. [/SIZE]
Monthly bank statements.
[SIZE="4"]If self employed, registration of company, business permits/license, taxes filed for previous year(s), bank statements and business income. [/SIZE]

等到拿到PR以后再去签美签可以签10年.

那了PR以后一般都是给10年。