I would appreciate any advice on which to purchase. I don't really understand why one would want the more expensive set. I'm just going to wire the officiant...

Sennheiser Evolution G3 100 Series - Wireless Microphone Combo (A / 516 - 558MHz) for $799

(Sennheiser Evolution G3 100 Series - Wireless EW 100 ENG G3-A -)

vs.

Sennheiser EW112-p G3 Camera Mount Wireless Microphone System For $599

(Sennheiser EW112-p G3 Camera Mount Wireless EW 112P G3-A - B&H)

The first includes two transmitters. One body pack with lav mic, the other is a transmitter that you can attach to a condenser mic turning that into a hand-held radio mic. So in effect it has two transmitters and one receiver, though you can only use one transmitter at a time on the same channel.
The second set is the body pack/lav mic and receiver only.

I see. So I'm guessing the basic set would be fine for me then. Just to my H4. Sorry for the ignorance, I'm just new to wireless..

The basic set will be fine, that's what I use. Here in UK it's not so acceptable to mic the officiant; they don't generally like to be wired, or they are using their own system.
I attach the mic to the groom and get him to put the transmitter into an inside pocket. That is usually good enough to capture the audio from groom, bride and officiant. It also means that I don't have to worry about making sure I get the pack back before the officiant goes, at least I know the grooms going to about all day.

The first includes two transmitters. One body pack with lav mic, the other is a transmitter that you can attach to a condenser mic turning that into a hand-held radio mic.

George, I know Dylan doesn't need the plug on transmitter but the one that does phantom power for condenser mics is the SENNHEISER WIRELESS SYSTEMS SKP-2000 (SKP2000 SKP 2000) (http://www.digibroadcast.com/products/11/SKP-2000_%28SKP2000_SKP_2000%29.htm) not the SKP 100 G3 which is the one supplied with the set quoted in his post.

Can't seem to get on to the Sennheiser UK website tonight for some reason.

Dylan, I don't use the system you're considering but what I would say is that event programme making is all about being prepared for the unexpected. Sooner or later you'll have a party that wants to pass a microphone around or uses a lectern, or has more speechmakers than you're equipped to handle, and you'll wish you paid a bit extra for the plug on transmitter.

For instance we discovered two days ago that there are to be five speeches at Friday's wedding - and the bride is thinking about singing a song a capella. We have four channels of radio mic but I am very pleased that a month ago (and for very different reasons) we invested in three AT boundary layer mics which will cover all Friday's needs very nicely.

Incidentally we get enough power out of our radios to provide phantom power for the boundary layer mics.

You,re right Colin, I should have said a dynamic mic.

Good advice Philip
I should have added that as well as the lav mic and body packs I also use SKM 100 handheld mic/transmitters. They comes in useful in situations where I can't get the groom to wear a mic. I can place it close to the ceremony. By setting a good balance of the sensitivity and receiver output it picks up surprisingly good audio even when not held close to the mouth. I keep it in my pocket so I can also use it as a pass-around for those who want to make informal or impromptu comments at the reception. It's a bit neater and lighter than a mic with the plug-on transmitter attached.

Wow, you guys are so generous with your suggestions. Thank you so SO much.