Sweetangel - I keep both species - and love keeping both species...
Firstly, most Monitor species don't handle well - in general neither of these species are particularly inclined to bite when handled, however, they generally remain flighty and skittish while being held...this, in conjunction with how blindingly fast they can be, means that they are likely to jump and do the runner...
I know of a few Ackies that will sit on a shoulder when out, however, the vast majority aren't quite some calm and controlled...
As far as captive animals go - both are AMAZING and settle in very well...
Ackies: The primary appeal over Gillens for me is the larger size and more striking colouration. Most look great and many look stunning. As young they are quite skittish for a while, however, will learn quite quickly that you mean food is coming...
Another thought is that Ackies will require a larger enclosure - I keep a pair in a 5x2x2 aquarium (150cm x 60cm metric) and would not go smaller - a single animal would not be kept long term in anything under 120cm by 60cm...and frankly, the larger the better - they are very active animals...
Gilleni: I have a trio of young Gillens who are amazing - I would tentatively say they have overtaken the Ackies as my favourite captive reptiles - they are personality PLUS...their major appeal for me is their bold and entertaining behaviour. Within a day of moving in they were boldly basking while I peered over the top of the tank - twisting their heads up to check me out. When the lid of the cricket container pops all their heads shoot out of the stack and peer up at me...
...and once the crickets start landing little grey torpedoes shoot from the stack at a lightning speed smashing down crickets...
These guys 'wag' their tail when excited, are as agile as monkeys jumping through the branches in their enclosure and have a look about them and head shape that looks just like a mini-perentie...
As you can no doubt tell - I am a HUGE fan of them - a very captivating animal...
Another benefit for yourself is enclosure size - you could comfortably house a pair of Gilleni in that enclosure - and the height gives the ability to put a few gnarled branches in there to get the most out of their agility displays...
Prices: Quite simply, they vary...this season Ackies ranged from $150 - 250 while Gillens ranged from $250 - 300. In saying that though, we saw much fewer small Monitors for sale last season than in the past and as such the prices may not be a true indication of a rate this season...
...and some generic advice - my first Ackie was a beautiful 'Sandfire' colouration WA animal - she was a cracker...however...I did make one mistake when buying her...I only bought ONE. To get the most entertainment and enjoyment from your small Monitors (particularly these two species) at least get a pair (if not more). Their antics and interactions with each other are often an absolute crack up...
I would also suggest buying
Reptiles Australia Magazine Vol 3 Iss 1 if you don't already have it...
Simon