For my exam I need to be able to state the differences between the two, so far I have...

NMJ are always excitatory, synapses can be inhibitory or excitatory
NMJ always use ACh, synapses can also use adrenaline and noradrenaline and other neurotransmitters.
A synapse is junction between two neurones, NMJ is junction between a neurone and a muscle.

Are these correct? Also are there any more?

Well done all of the points you make are correct. I can't think of any others obvious other than a synapse can be part of a multi neuronal pathway where as the NMJ is by definition the end of a pathway. Good luck with the exam.

Off-hand a couple of other points:
(1) the NMJ is a synapse (a junction between an axon terminal and receiving cell);

(2) most classical neurotransmitters such as serotonin and dopamine are taken back into the presynaptic neuron by re-uptake; Ach is removed from the synapse by breakdown by an enzyme called acetylcholinesterase;

(3) apart from the classical neurotransmitters there are many other chemicals such as neuropeptides, cannabinoids, etc. that modulate function at the synapse, acting at the pre-synaptic and/or post-synaptic neuron. Often a pre-synaptic/post-synaptic role is mediated by different receptors from the same neurotransmitter receptor family;

(4) Ach at the skeletal muscle NMJ acts on nicotinic Ach receptors (ligand-gated ion channels); Ach at the smooth/cardiac muscle NMJ acts on muscarinic Ach receptors (these are G protein-coupled receptors).