AS the stock market once again hit record heights this week, investors are asking if prices can really continue to rise.

It is, of course, a conundrum that's cropped up many times, as the FTSE-100 - better known as the Footsie - crashed through the 4000 barrier, raced on to 5000 and is now regularly breaking through at 6000.

The market may well hit headier highs - but what if this really is the end of the road?

Is this a sensible time to be committing a large lump sum to the stock market? One answer is to put your money in the hands of a stockbroker. They won't always get it completely right, but they will have access to statistics which should help them make the sort of informed decisions that are beyond most private investors.

But there's another answer if you don't have the sort of money to interest a broker, and that's to move money into shares on a little and often basis. The most sensible way is to put it into a unit or investment trust savings plan.

These funds put your monthly contribution with those of hundreds of other investors to create a huge pool of shares, here and overseas. This reduces the risk that you'll pick a handful of duds and, because your money goes in regularly, reduces the impact of poor timing.

In fact, if the market was to fall, investing monthly can work to your advantage. A fixed amount going into the fund buys more units or shares as prices fall - so you have more to benefit from rising prices when the market starts to go up again.

So should you choose unit or investment trusts? According to Money Management Magazine figures, the answer is clear.

Regular savings in the top performing investment trusts have beaten units over every period the magazine analysed.

Recent investment trust performance figures are spectacular, being dominated by split capital and warrants, both highly volatile and among the riskiest sectors. Indeed, there have been calls for these to be removed from the performance tables for fear of giving people the wrong idea of what to expect.

But over a longer period, generalist and much more conservative investment trusts, investing here and internationally, have beaten their unit trust counterparts. What's more, initial costs on units can be as high as 6%, but don't exceed 1.8% for investment trusts, although yearly fees can be higher for investment trust savings schemes.

''There are signs the investment trust sector is changing for the better and the over-supply of recent years is being reduced,'' says Roderic Primrose of East Lothian stock-

broker McInroy & Wood, which has just launched a unit trust with no initial charge investing only in investment trusts.

''This is already limiting the discount of the share price of trusts to their asset values - this has narrowed from around 12% before Christmas to 10% now - and there is every reason to believe this will continue, enhancing returns.''

Investing long term in an investment trust also beats another option - a with-profits endowment. These are not directly comparable - you get life insurance and protection from market crashes with a with-profits policy and there's no potential extra tax liability, unlike with a unit or investment trust plan. But performance differences are startling.

Saving #50 a month in the top performing investment trust over 20 years has produced well over #150,000, while the best with-profits plan managed just under #60,000.

You can get details of savings plans from the Unit Trust Information Service, 65 Kingsway, London WC2 6TD; 0181 207 1361, and the Association of Investment Trust Companies, Durrant House, 8-13 Chiswell Street, London EC1Y 4YY.