As Hagster said you need to work out your link budget, but with if I understand your configuration correctly 2 watts available then there should be no problem with a smal antenna. When deciding on an antenna to use there is more than just the received power to consider and every on ewill have differing criteria so ther may not be a single right answer for every situation. A yagi may be larger than a small whip antenna but does give some advantages. Directivity means that your receiver will be potetially subject to less interfence, off axis signals will be supressed though there may be some reflections from the buildings. The antenna gain means that your transmit power can be reduced, lowering both initial cost and operating cost, and again the interference potential to other users is reduced by theerduction in power and the directivity of the antenna.
If the TX & RX or antennas are to be mounted at 30m, then the practicalities of mounting either type of antenna are about the same. Mounting structure cables and riggers wil dominate over the actual antenna.
From a theoretcal exercise point of view given your power and assuming a reasonable receiver the proverbial piece of wet string will do, from a practical poit of view even though it may seem overkill, assuming regulations allow it I'd go for the directional antenna, a 10 or 12dBi yagi is not that big at 400MHz; at 2 meters long you would not want to cary it about but mounted on a building or tower it should be no problem.