Big difference, just a fraction of one db gain can mean the difference between nothing at all and a watchable picture.
Be aware, however, that some manufacturer's figures are absolute maxima, measured under perfect conditions, and the real gain in the field can be lower.
The most obvious example of this is with FortecStar, who promise a gain of 48.4 on their 2.4m prime focus. This is at a frequency of 12.5 ghz, which is at the highest end of the bands a dish can receiver. Most honest manufacturers give the value at mid-band, 11.5ghz, a lower frequency which returns a lower gain figure. I know from practical experience this particular FortecStar dish delivers nothing like that.
Most footprint maps for Spain will show many areas such as mine as needing a 2.2 to 2.4 metre antenna. You can get a reasonable result with a 1.8 metre in these areas, but it has to be a top quality fibre based dish such as ChannelMaster or Prodelin 1.8m, which both have a midband gain of 45.5. The 2.4 metre ChannelMaster Offset has a gain of 47.6.
I know of someone in Madrid with the 2.0 metre IRTE who has performance similar to the Gibertini 1.5 metre. This could be down to choice of LNB or receiver, however. She loses BBC2 early to mid- evening.
Famaval (so- called "Portugese dish") have a similar gain to IRTE.
All prime focus dishes have lower gain than offset dishes of the same size.
The difference in practical terms between a Gibertini 1.5m and a Prodelin 1.8 m is about 3 to 4 hours more viewing on BBC 2 round here.
You have to take a view of the best cost / size / gain balance.
The very best dishes are pretty expensive, but have professional performance ( Andrew/ChannelMaster are used by broadcasters) and are mechanically more robust.
Remember many dealers and installers will try to sell you a lower cost dish as they are dirt cheap at trade prices.
Where are you located?