TORONTO, April 17 (Reuters) - Two prominent proxy advisers have split on whether shareholders should vote to maintain Augusta Resource Corp's shareholder rights plan, which could thwart a hostile takeover bid from larger Canadian base metals miner HudBay Minerals Inc.

Glass Lewis has advised its clients to vote to preserve the plan, while rival Institutional Shareholder Services (ISS) has come out against the plan. ISS also sharply criticized the amount of severance that could be paid to Augusta's chief executive if HudBay's bid succeeds.

Investors will vote on whether to cancel the rights plan at Augusta's May 2 shareholder meeting. Without the plan, HudBay's bid, which expires May 5, would be more likely to succeed.

Shareholder rights plans, often called poison pills, are designed to make hostile takeovers difficult.

HudBay wants control of Augusta's Rosemont project in Arizona, seen as one of the most promising copper projects in the United States. Augusta said in March that nine parties had expressed some interest in the company.

In its report, which was viewed by Reuters, Glass Lewis said it believes that rights plans are generally "not conducive to good corporate governance," but that it supports limited plans in some circumstances.

Poison pills can give directors more time to seek out friendly bidders, boosting payouts to investors.

Glass Lewis said Augusta's plan does not include any of several provisions that would normally raise governance concerns. For example, the plan does not stipulate that buyers must pay cash, as some plans do.

But in its report, also seen by Reuters, ISS criticized many features of the plan, including a provision under which the plan kicks in if an investor acquires 15 percent or more of the company, and the fact that it does not allow partial bids.

ISS said the size of the severance Augusta could pay its CEO if there is a change in control is "truly egregious in the Canadian market," and thus investors should withhold votes from the three directors that serve on the Augusta board's compensation committee.

If the chief executive resigns for any reason after a change in control, Augusta would pay four times base salary and a target bonus as severance. ISS said two times salary and bonus is best practice in Canada.

Glass Lewis advised clients to withhold votes from another director, Lenard Boggio, saying he serves on too many audit committees.

HudBay has asked securities regulators to cease trade the poison pill, which would render it ineffective regardless of the outcome of the shareholder vote.

"Our intention is to put the power of this important decision in the hands of Augusta shareholders by giving them the opportunity to vote on the rights plan on May 2, three days before the expiry of HudBay's bid," Augusta Chief Executive Gil Clausen said in a release.

Proxy advisory firms such as Glass Lewis and ISS advise institutional investors ahead of shareholder votes. Their reports often influence votes for or against management.

(Editing by Peter Galloway)