The accurate simulation of polydisperse sprays encountering coalescence in unsteady gaseous flows is a crucial issue for solid rocket booster optimization. Indeed, the internal flow of the engine depends strongly on the alumina droplet size distribution, which spreads up with coalescence. Yet solving for unsteady two-phase flows with a high dimensional phase space is a challenge for both modelling and scientific computing. The usual Lagrangian approaches lead to a very high computational cost or to a low resolution level and they induce coupling difficulties to the Eulerian gaseous phase description. A wide range of Eulerian models have been recently developed to describe the disperse liquid phase at a lower cost and with an easier coupling to the carrier gaseous phase. Among these models, the Multi-Fluid model allows the detailed description of polydispersity and size/velocity correlations by separately solving fluids of size-sorted droplets, the so-called sections. On the one hand, the existing first order description of the size distribution in each section provides simple and fast resolution for coalescence. On the other hand, a second order method allows to reduce the number of sections required to capture accurately coalescence and to use elaborate droplet collision modelling, yet at the cost of heavier computation algorithms. This paper seeks to conclude on computational time and precision of both methods in order to choose the most efficient configuration for multi-dimensional unsteady rocket chamber simulations. Its objective is threefold: first, to validate the second order method by comparing simulations to reference solutions and dedicated experimental measurements conducted at ONERA, second to study the efficiency and robustness of both methods on coalescing size-conditioned dynamics problems, third, to draw some firm conclusions about the necessity to use first order or second order methods in order to capture the physics of solid propulsion configurations.