Units do not fire through friendlies, weapons create craters, explosive weapons more noticeably so, and i am not sure what you mean about impact. Perhaps impulse? Explosions create a shockwave which physically moves units, if that is what you mean (try a roach in the middle of some peewees

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Tbh OTA had some serious balance problems, about 65% of the units were useless, and stuff like line bombing/hawk dancing/lul ur mizzile dnt hit mai pelicans on wator!1 make me feel that making direct comparisons in terms of cost etc isn't really appropriate.
Off the top of my head, here are some of the major differences which may catch you out:
T1 gunships, they can drop a com in seconds, do not equate these to ba/ca t1 gunships they are far more lethal
Sumo/Golly are massively exaggerated and in both cost and effectiveness compared to ota
Freaker and Zipper are t1, and flea is t2 (awesome swarm unit)
AA bots target ground, they function much like samson/slasher (flak also targets ground, high ROF and good AOE make them great anti swarm)
Leveller is a t2 flame tank
Warrior is t2, and an excellent skirmisher/raider
Both F.A.R.K. and Necro can resurrect, have extremely powerful nanolathes, and a limited build menu (Necro has access to greatly superior defences than F.A.R.K.)
Spiders fire a constant beam, as opposed to a single shot, making them much more viable, although they require a fair amount of micro to be effective
Penetrator fires a constant beam (think ota one fired like a sniper, just with a really shitty aim speed)
Antinuke is cloakable
LLTs are a lot more expensive than in b/ca and dont really stand up to a t1 force of anything other than a few peewee/ak at all, they are point defense and nothing more.
Moho mines cost 300e/tick to run
Core get access to the minifusion with t2 builders, whereas Arm get the mobile fusion from the t2 vehicle lab. This is the crux of the faction differention, and marks quite a departure from ota tbh. In order of total energy produced (and cost): mini/mobile fus --> Cloakable fus --> Full fus
Core has a vastly superior economy, as they get necros and minifusion from 1 lab, and also have better defensive structures, the annihilator aside. Arm are far more mobile than core, and have much better raiding units than core does (fleas, warriors and panthers). Arm t2 kbots are extremely powerful and versatile, but it is difficult to maintain an adequate energy output to power the mohos necessary to build them without access to mobile fusions; you are forced to invest a lump sum into a cloakable or full fusion, or mass vunerable t1 economy. Your ability to grow exponentially is drastically impaired by going t2 Arm bots, you really need to push for a quick kill if you decide to do this.
Hrm this post is getting way longer than I intended, suffice to say XTA places more emphasis on intelligent choice and use of units than on scrabbling to outproduce your opponent and smash into his llts with a horde of t1 units. The reason i would suggest it is closer to OTA than other derivative mods is that it doesnt include a host of uberhack units and structures: arm have no t3, there are no shield generators, commandos, nuke bombers etc, and this coupled with the lack of a multitude of armour classes gives a far greater degree of "what you see is what you get" in your units.
Sry for long post