I'm thrilled with my T|X. Haven't really used a Lifedrive, other then playing with one in the Palm Store near my house.
The T|X will be closer to your 515 in size, if that matters. The lifedrive is larger and bulkier then the T|X, which was a big factor for me.
Also, you can always purchase memory cards for the T|X to give you more storage. i currently am using a 1mb card, but I know they have 2 mb cards, and I believe the T|X will also support a 4mb card, but I'm not positive about that.
When I was setting up my T|X for the first time, it asks what language I want to use, and gives a long list. I am sure German is on the list, but again, I don't remember seeing it, but I'd be very surprised if it wasn't. I'd bet either the LD or T|X can be used in german.
My suggestion would be, get the T|X, save some money, and put the savings into memory cards, which are coming down quickly in price!
If you have an electronics store near you, you should try to go and play with both devices. Maybe the size and weight of the Lifedrive won't bother you as much as it did me. I carry my Palm around every day, and want it as small and sleek as I can get it!
I'm sure you'll love either one! Big improvement over what you currently are using!
(Built in wifi is really a blast! Love being able to download mail, the weather, browse websites to read the news while sitting at my local coffee shop, etc.)