If you ordered from RCCommand you should get a couple of spare links and pins with each the tanks - only the non smoke and sound tanks come without spares, and I don't think Bill stocks that rubbish.

You should avoid long grass with the Pershing as it tends to get tangled in the drive sprockets, causing the motor to stall and potentially blowing the control board - they aren't too expensive too to replace (~$20), but best to avoid doing it in the first place
As Bob already said, the Tiger tracks aren't all that strong but I've not had any problems with either of my Pershings. The same goes for the track pins, the Tigers tend to loose their's so inspect them regularly if you don't want to spend time hunting them in the back yard
The tiger has a bad reputation as the early ones were pretty poorly designed, but most of the mechanical flaws have been fixed (or atleast they aren't as bad as they were).

Don't expect too much scale accuracy from them and you will be happy with them. There are a lot of upgrades out there for both of them if you want to add more detail, better performance, or get involved in club level IR battles - just be aware that if you go too far down this road you will get to the point where it makes more sense to buy a Tamiya kit than keep adding all of the 'whistles and bells' that are out there.

Welcome to the money pit!
Mart