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The extra gain will make a bit of difference but not a great deal as it is still an omin directional . Difference is it will have a slightly lower angle of coverage. Best if you can get it as high as possible.
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It sounds like somebody is trying to sell you an new aerial.
There are two possible reasons why you might need a bigger aerial: 1. The receiver in your new radio is less sensitive than the old one. Surely this is unlikely? 2. The transmitter has less power output so you need a bigger aerial to compensate for this loss. This may be more likely.
Omnidirectional VHF aerials can still have gain. They achieve this by sending the signal out parallel to the ground and by reducing the amount which goes upwards. (and the same but in reverse for reception).
Decibels (db) are a logarithmic scale. A gain of 3db is the equivalent of doubling the transmitter power.
The rule of thumb with VHF aerials is that the higher they are and the bigger they are, the better they will work.