My bunch of cents-
What I think is very important thing they have in common is that they both are INTERNAL.It seems to be that japanese and chinese approach to Ki/Chi is a bit different.
Tai chi is a lot like qi-gong and works that way (slow motion solo movements etc. great health benefits) Even though aikidokas talk very much about ki if compared to various other japanese arts like judo,jujutsu (at least to a point) or karate (again,IĀ“d say to a point because my idea is that it comes to play later and they donĀ“t slam it into your face as in aikido

)
Aikido has some qi-gong in it though,founder built his own kind of short qi-gong set which includes "rowing exercise" and such.
Tai Chi Chuan players definitely strike&kick more than aikidoists,that is an essential part as far as my limited knowledge goes.To tell you the truth,this is also an "original" part of aikido,but is often neglected in modern aiki,not to say it would be completely lost though (AND it is used to follow-up with a throw,lock whatever they come up with.It is called atemi,used to soften up) Aikido also does not do much solo kata,except weapon forms maybe.
In tai chi there is a great amount of pressure point techniques in it,this is not so much of a big thing in aikido (but some pressure points for joint manipulation,some for strikes)
I also feel that while aikido is theoretically meant to be non-harmful in application,causing from none to minimal injury,tai chi then,has more deadly potential to it (as dim mak/pressure points previously mentioned,you might want to consider this a brief point though for it is hard to find an art as peaceful in application as aikido but schools and teachers vary)
Aikido is completely defensive in application (sure you could just grab someoneĀ“s wrist and shake them to a throw,and if it works thatĀ“s allright but generally,it would not fit to principles as you would have to borrow opponentĀ“s force,especially if youĀ“re physically lower) while I bet a tai chi person could throw a heel kick or two without turning tables on his/her art (please note though that this is more like my own understanding as I have not trained in TCC) also remembering that chinese MA is basically defensive.
This is some things I consider worthy and I wrote this for a long time...there may be more.
I hope this does help some.
:asian: