Well, I can't give you concrete guidance... if the panel has a component input by default, and it costs more for SCART, then use the component input, unless you need it for something else.
HDMI/DVI is only useful for either a true HD source (Sky HD, a PC with HD playback, or cable HD, and in future HiDef DVD formats, or HD via Freeview; don't hold your breath for that) or for a pseudo-HD source, which is something like my DVD player, which can upscale a standard picture to 720p or 1080i via HDMI or DVI.
Component is useful because on most panels it can accept an HD source, or a progressive-scan SD source (which can be better for DVDs), and it is possible, depending on the equipment to get slightly better picture via component than via SCART, though the difference may be barely noticeable.
Your main aim in this is simply to get the best picture from each source, and to ensure that the kit you have now, and in the future, can also be connected in such as way as to get the best picture.
The Toppy is only SD, and interlaced, but it can produce component; that really just means you have slightly more options in terms of connection choices.
If it's the only source you're connecting to the panel, it really doesn't matter - SCART or component will be pretty much identical, and you should choose based on what cables you have to hand and what inputs the panel has for you to use.
It's only when you have sources that can potentially produce better quality pictures that you need to worry about what goes where. I'd be tempted to get a panel with 2 HDMI sockets, frankly; one for a DVD and one for something else with HD in future. Then use whatever's left over for the Toppy.
But really, it's impossible to tell you the right answer without the details of all your kit and, with the best will in the world, that's not the most exciting way for either of us to spend a Sunday afternoon.
Nigel.