The Palestinian parliament-in-exile opens a five-day session here today to decide whether Palestinians will join or boycott the planned Middle East peace conference.The meeting of the 451-member Palestine National Council (PNC) is expected to see fierce argument over the value of sending a delegation to the talks, even as United States Secretary of State James Baker III prepares to issue invitations to the conference, scheduled for October. Egypt, Syria, Lebanon, Jordan have all said they will attend the meeting, and Israel has said it will do so too, on certain conditions. Attention is thus focused on the Palestinian response. "The Palestinians are facing their most difficult and critical situation since the PLO was established" nearly 30 years ago, says Mohammed Milhem, a member of the Palestine Liberation Organization's executive committee. Some Palestinians will agree with Mr. Baker that this opportunity to reach a settlement with Israel may not repeat itself for years. "The bus may not come by again for a long, long time," Baker warned last week in Jordan. "I believe the Palestinians have most to lose" by not attending the conference "because they have most to gain" if they do participate, he argued. Palestinians in favor of joining the talks, most likely as part of a joint delegation with Jordan, fear that if they do not strike a deal with Israel now over the future of the occupied territories, the Jewish state will house so many settlers in the West Bank and Gaza Strip that there will soon be nothing left to negotiate about. Others are more skeptical of the proposed conference, doubting that it would lead to an Israeli withdrawal from the territories, and fearing it would merely seal Palestinian concessions on key issues.