Transfer Benefits
If you take the paid-up route from a Defined Benefit plan you will be promised a specific benefit (related to your years of service and current salary) at normal retirement age. This deferred pension (paid-up pension) will be indexed up to retirement at the lower of 4% p.a. or CPI in order to maintain the real value.
This is the safe option as you have reasonable certainty as to what you will get on reaching retirement age. Mercer should be able to tell you what that pension is in today's terms and you can indexed it forward to retirement (at say 3% p.a.) to give you the estimated pension at say 60 or 65.
As an alternative to this route, the Trustees will also offer a transfer value (a cash amount) which you could invest in either a Buy Out Bond of another pension plan. This is a riskier option since it is more difficult to estimate what this might grow to at 60 or 65 and then to estimate what pension this fund would buy at that stage. If the Transfer Value was invested well, it could do better than the deferred pension or it could do worse.
It it impossible to say which is the best route. The deferred pension route is the safest less risky option as you can reasonably estimate what your pension will be at say 60 or 65 (because you will know exactly what it is in today's terms). The Transfer Value route offers the potential to get a better return by investing a cash sum in lieu of the deferred pension, but equally it could do worse depending on how the transfer value is invested.
So on balance it is probably a decision which is based on your attitude to risk, the deferred pension being the safe bet, the transfer value being the gamble.
But get Mercer to explain all the options. That's what they are paid to do.