For the 2 million civil servants who must pick a pension plan this year, there are firms that -- for a fee -- will help them decide. They will provide a detailed computer printout -- based on a questionnaire filled out by the individual worker -- showing what the benefits would be under the Civil Service Retirement Systems as opposed to what the benefits would be by switching to the Federal Employees Retirement System.

By Dec. 31 most federal workers hired before 1984 must decide whether to remain in the old CSRS retirement program or move into FERS, the new plan. Those who leave the CSRS plan cannot come back to it.

Benefits under the old system are based on salary and length of service. Retirees are guaranteed annual raises to keep them current with inflation.

Under the new FERS system, workers get smaller civil service benefits and a less generous cost-of-living adjustment formula. But FERS offers bigger and better tax-deferred investment options and more pension flexibility for workers who leave the government.

About 300,000 workers here face the pension decision. Many are buying computer printouts that show their benefit levels under both plans. Some find the information invaluable. Others say it is confusing.

Before signing up for one of the services, here are some things workers should consider:They may be able to get the same service free through their agency or get it free or at reduced rates through their union or professional association.The Office of Personnel Management has produced a free 124-page publication (workers should have received it already at the office) called the FERS Transfer Handbook. It explains both pension plans, lists advantages and disadvantages and describes how to switch.

OPM's handbook also has worksheets that enable you to do much of the pension comparison work. OPM also has trained employes in other agencies. Most officials believe that the guidance available from the government, for free, is sufficient to help workers make the choice on what to do.

A number of Washington area firms now offer computer printout services that will show benefits workers can expect by remaining with the current pension plan versus those available by moving into the new FERS program. In most cases workers will be asked to fill out a detailed questionnaire telling such things as age, salary, when they plan to retire and how much they will invest in the thrift plan.

Firms offering the computer pension comparisons are:GRB Inc., 5400 Eisenhower Ave., Alexandria, Va., 22304. Telephone 461-9100. Cost of the analysis is $25.MAPS/Chambers Associates. P.O. Box 65594, Washington, D.C., 20035. Telephone 857-0670. Cost of the analysis, $25.PIPS Inc., P.O. Box 477, Falls Church, Va., 22044. Telephone 532-1631. Fee for the analysis is $32.Sort Inc., 3826 Gallows Rd., Annandale, Va., 22003. Telephone 354-8337. Fee for the printout is $32.American Financial Services, 6560 Backlick Rd., Springfield, Va., 22150. Telephone 451-1300. Cost of the analysis is $12.Dunstan Pension Information Services, 2025 I St. NW., Washington, D.C., 20066, says it has a $39.95 computer program that can be used to compare benefits under both plans. The telephone number is 340-9818.