A bit hard to compare, as nobody has had a chance to play with not-Menasor yet, but assuming that both are at least of equal engineering marvel, I'd go with FP not-Menasor for sentimental value and hedging against price-appreciation.
First, there's that added sentimental value from your childhood. Secondly, none of Fansproject's releases has fallen in value after initial release (the least popular was probably the not-Insecticons, but they were KO'ed and the original moulds still remained the same price), so it's only by pre-ordering that you will secure the lowest price. The second FP not-Bruticus set will likely remain the same price as the second release has ruined scalpers' profits, giving you some time to save up.
However, price-wise not-Menasor will cost you more as the second release of FP not-Bruticus is $260 ($130+130), whilst for the full set of FP not-Menasor will be upwards of $~360 (first two figures are $60each, whilst second two limbs will likely be the same price + not-Motormaster who will be at least Voyager sized). Noting my second reason, I'd rather not risk the chance of having to pay the scalper's rate at a later stage. Also, scalpers seen at work here: http://www.ebay.com.au/itm/Transform...#ht_2105wt_909